NAG Library Chapter Introduction

E04 (opt)
Minimizing or Maximizing a Function


2  Background to the Problems

Scope of the Chapter

This chapter provides routines for solving various mathematical optimization problems by solvers based on local stopping criteria. The main classes of problems covered in this chapter are:
For a full overview of the functionality offered in this chapter, see Section 5 or the Chapter Contents (Chapter E04).
See also other chapters in the Library relevant to optimization:
This introduction is only a brief guide to the subject of optimization designed for the casual user. It discusses a classification of the optimization problems and presents an overview of the algorithms and their stopping criteria to assist choosing the right solver for a particular problem. Anyone with a difficult or protracted problem to solve will find it beneficial to consult a more detailed text, see Gill et al. (1981), Fletcher (1987) or Nocedal and Wright (2006). If you are unfamiliar with the mathematics of the subject you may find Sections 2.1, 2.2, 2.3, 2.6 and 3 a useful starting point.

Background to the Problems

Introduction to Mathematical Optimization

Mathematical Optimization, also known as Mathematical Programming, refers to the problem of finding values of the inputs from a given set so that a function (called the objective function) is minimized or maximized. The inputs are called decision variables, primal variables or just variables. The given set from which the decision variables are selected is referred to as a feasible set and might be defined as a domain where constraints expressed as functions of the decision variables hold certain values. Each point of the feasible set is called a feasible point.
A general mathematical formulation of such a problem might be written as
minimize fx subject to xF  
where x denotes the decision variables, fx the objective function and F the feasibility set. In this chapter we assume that Fn. Since maximization of the objective function fx is equivalent to minimizing -fx, only minimization is considered further in the text. Some routines allow you to specify whether you are solving a minimization or maximization problem, carrying out the required transformation of the objective function in the latter case.
A point x* is said to be a local minimum of a function f if it is feasible (x*F) and if fxfx* for all xF near x*. A point x* is a global minimum if it is a local minimum and fxfx* for all feasible x. The solvers in this chapter are based on algorithms which seek only a local minimum, however, many problems (such as convex optimization problems) have only one local minimum. This is also the global minimum. In such cases the Chapter E04 solvers find the global minimum. See Chapter E05 for solvers which try to find a global solution even for nonconvex functions.

Classification of Optimization Problems

There is no single efficient solver for all optimization problems. Therefore it is important to choose a solver which matches the problem and any specific needs as closely as possible. A more generic solver might be applied, however, the performance suffers in some cases depending on the underlying algorithm.
There are various criteria to help to classify optimization problems into particular categories. The main criteria are as follows:
Each of the criterion is discussed below to give the necessary information to identify the class of the optimization problem. Section 2.5 presents the basic properties of the algorithms and Section 3 advises on the choice of particular routines in the chapter.

Types of objective functions

In general, if there is a structure in the problem the solver should benefit from it. For example, a solver for problems with the sum of squares objective should work better than when this objective is treated as a general nonlinear objective. Therefore it is important to recognize typical types of the objective functions.
An optimization problem which has no objective is equivalent to having a constant zero objective, i.e., fx=0. It is usually called a feasible point problem. The task is to then find any point which satisfies the constraints.
A linear objective function is a function which is linear in all variables and therefore can be represented as
fx= cTx+c0  
where cn. Scalar c0 has no influence on the choice of decision variables x and is usually omitted. It will not be used further in this text.
A quadratic objective function is an extension of a linear function with quadratic terms as follows:
fx= 12 xTHx+ cTx .  
Here H is a real symmetric n×n matrix. In addition, if H is positive semidefinite (all its eigenvalues are non-negative), the objective is convex.
A general nonlinear objective function is any f:n without a special structure.
Special consideration is given to the objective function in the form of a sum of squares of functions, such as
fx= i=1m ri2x  
where ri:n; often called residual functions. This form of the objective plays a key role in data fitting solved as a least squares problem as shown in Section 2.2.3.

Types of constraints

Not all optimization problems have to have constraints. If there are no restrictions on the choice of x except that xF=n, the problem is called unconstrained and thus every point is a feasible point.
Simple bounds on decision variables xn (also known as box constraints or bound constraints) restrict the value of the variables, e.g., x510. They might be written in a general form as
lxi xi uxi , for ​ i=1,,n  
or in the vector notation as
lx x ux  
where lx and ux are n-dimensional vectors. Note that lower and upper bounds are specified for all the variables. By conceptually allowing lxi=- and uxi=+ or lxi=uxi full generality in various types of constraints is allowed, such as unconstrained variables, one-sided inequalities, ranges or equalities (fixing the variable).
The same format of bounds is adopted to linear and nonlinear constraints in the whole chapter. Note that for the purpose of passing infinite bounds to the routines, all values above a certain threshold (typically 1020) are treated as +.
Linear constraints are defined as constraint functions that are linear in all of their variables, e.g., 3x1+2x24. They can be stated in a matrix form as
lB Bx uB  
where B is a general mB×n rectangular matrix and lB and uB are mB-dimensional vectors. Each row of B represents linear coefficients of one linear constraint. The same rules for bounds apply as in the simple bounds case.
Although the bounds on xi could be included in the definition of linear constraints, it is recommended to distinguish between them for reasons of computational efficiency as most of the solvers treat simple bounds explicitly.
A set of mg nonlinear constraints may be defined in terms of a nonlinear function g:nmg and the bounds lg and ug which follow the same format as simple bounds and linear constraints:
lggxug .  
Although the linear constraints could be included in the definition of nonlinear constraints, again we prefer to distinguish between them for reasons of computational efficiency.
A matrix constraint (or matrix inequality) is a constraint on eigenvalues of a matrix operator. More precisely, let 𝕊m denote the space of real symmetric matrices m by m and let A be a matrix operator A:n𝕊m, i.e., it assigns a symmetric matrix Ax for each x. The matrix constraint can be expressed as
where the inequality S0 for S𝕊m is meant in the eigenvalue sense, namely all eigenvalues of the matrix S should be non-negative (the matrix should be positive semidefinite).
There are two types of matrix constraints allowed in the current mark of the Library. The first is linear matrix inequality (LMI) formulated as
Ax= i=1 n xi Ai - A0 0  
and the second one, bilinear matrix inequality (BMI), stated as
Ax= i,j=1 n xi xj Q ij + i=1 n xi Ai - A0 0 .  
Here all matrices Ai, Qij are given real symmetric matrices of the same dimension. Note that the latter type is in fact quadratic in x, nevertheless, it is referred to as bilinear for historical reasons.

Typical classes of optimization problems

Specific combinations of the types of the objective functions and constraints give rise to various classes of optimization problems. The common ones are presented below. It is always advisable to consider the closest formulation which covers your problem when choosing the solver. For more information see classical texts such as Dantzig (1963), Gill et al. (1981), Fletcher (1987), Nocedal and Wright (2006) or Chvátal (1983).
A Linear Programming (LP) problem is a problem with a linear objective function, linear constraints and simple bounds. It can be written as follows:
minimize xn cTx subject to lBBxuB lxxux  
Quadratic Programming (QP) problems optimize a quadratic objective function over a set given by linear constraints and simple bounds. Depending on the convexity of the objective function, we can distinguish between convex and nonconvex (or general) QP.
minimize xn 12 xTHx + cTx subject to lBBxuB lxxux  
Nonlinear Programming (NLP) problems allow a general nonlinear objective function fx and any of the nonlinear, linear or bound constraints. Special cases when some (or all) of the constraints are missing are termed as unconstrained, bound-constrained or linearly-constrained nonlinear programming and might have a specific solver as some algorithms take special provision for each of the constraint type. Problems with a linear or quadratic objective and nonlinear constraints should be still solved as general NLPs.
minimize xn fx subject to lggxug lBBxuB lxxux  
Semidefinite Programming (SDP) typically refers to linear semidefinite programming thus a problem with a linear objective function, linear constraints and linear matrix inequalities:
minimize xn cTx subject to   i=1 n xi Aik - A0k 0 ,  k=1,,mA lBBxuB lxxux  
This problem can be extended with a quadratic objective and bilinear (in fact quadratic) matrix inequalities. We refer to it as a semidefinite programming problem with bilinear matrix inequalities (BMI-SDP):
minimize xn 12 xTHx + cTx subject to   i,j=1 n xi xj Qijk + i=1 n xi Aik - A0k 0 ,  k=1,,mA lBBxuB lxxux  
A least squares (LSQ) problem is a problem where the objective function in the form of sum of squares is minimized subject to usual constraints. If the residual functions rix are linear or nonlinear, the problem is known as linear or nonlinear least squares, respectively. Not all types of the constraints need to be present which brings up special cases of unconstrained, bound-constrained or linearly-constrained least squares problems as in NLP .
minimize xn i=1mri2x subject to lggxug lBBxuB lxxux  
This form of the problem is very common in data fitting as demonstrated on the following example. Let us consider a process that is observed at times ti and measured with results yi, for i=1,2,,m. Furthermore, the process is assumed to behave according to a model ϕt;x where x are parameters of the model. Given the fact that the measurements might be inaccurate and the process might not exactly follow the model, it is beneficial to find model parameters x so that the error of the fit of the model to the measurements is minimized. This can be formulated as an optimization problem in which x are decision variables and the objective function is the sum of squared errors of the fit at each individual measurement, thus:
minimize xn i=1mri2x where rix = ϕti;x -yi  

Problem size, dense and sparse problems

The size of the optimization problem plays an important role in the choice of the solver. The size is usually understood to be the number of variables n and the number (and the type) of the constraints. Depending on the size of the problem we talk about small-scale, medium-scale or large-scale problems.
It is often more practical to look at the data and its structure rather than just the size of the problem. Typically in a large-scale problem not all variables interact with everything else. It is natural that only a small portion of the constraints (if any) involves all variables and the majority of the constraints depends only on small different subsets of the variables. This creates many explicit zeros in the data representation which it is beneficial to capture and pass to the solver. In such a case the problem is referred to as sparse. The data representation usually has the form of a sparse matrix which defines the linear constraint matrix B, Jacobian matrix of the nonlinear constraints gi or the Hessian of the objective H. Common sparse matrix formats are used, such as coordinate storage (CS) and compressed column storage (CCS) (see Section 2.1 in the F11 Chapter Introduction).
The counterpart to a sparse problem is a dense problem in which the matrices are stored in general full format and no structure is assumed or exploited. Whereas passing a dense problem to a sparse solver presents typically only a small overhead, calling a dense solver on a large-scale sparse problem is ill-advised; it leads to a significant performance degradation and memory overuse.

Derivative information, smoothness, noise and derivative-free optimization (DFO)

Most of the classical optimization algorithms rely heavily on derivative information. It plays a key role in necessary and sufficient conditions (see Section 2.4) and in the computation of the search direction at each iteration (see Section 2.5). Therefore it is important that accurate derivatives of the nonlinear objective and nonlinear constraints are provided whenever possible.
Unless stated otherwise, it is assumed that the nonlinear functions are sufficiently smooth. The solvers will usually solve optimization problems even if there are isolated discontinuities away from the solution, however, it should always be considered if an alternative smooth representation of the problem exists. A typical example is an absolute value xi which does not have a first derivative for xi=0. Nevertheless, it can sometimes be transformed as
xi= xi+- xi- , xi= xi++ xi- , where xi+ , ​ xi- 0  
which avoids the discontinuity of the first derivative. If many discontinuities are present, alternative methods need to be applied such as e04cbf or stochastic algorithms in Chapter E05, e05saf or e05sbf.
The vector of first partial derivatives of a function is called the gradient vector, i.e.,
fx = fx x1 , fx x2 ,, fx xn T ,  
the matrix of second partial derivatives is termed the Hessian matrix, i.e.,
2 fx = 2fx xixj i,j=1,,n  
and the matrix of first partial derivatives of the vector-valued function f:nm is known as the Jacobian matrix:
Jx = fix xj i=1,,m,j=1,,n .  
If the function is smooth and the derivative is unavailable, it is possible to approximate it by finite differences, a change in function values in response to small perturbations of the variables. Many routines in the Library estimate missing elements of the gradients automatically this way. The choice of the size of the perturbations strongly affects the quality of the approximation. Too small perturbations might spoil the approximation due to the cancellation errors in floating-point arithmetic and too big reduce the match of the finite differences and the derivative (see e04xaf/e04xaa for optimal balance of the factors). In addition, finite differences are very sensitive to the accuracy of fx. They might be unreliable or fail completely if the function evaluation is inaccurate or noisy such as when fx is a result of a stochastic simulation or an approximate solution of a PDE.
Derivative-free optimization (DFO) represents an alternative to derivative-based optimization algorithms. DFO solvers neither rely on derivative information nor approximate it by finite differences. They sample function evaluations across the domain to determine a new iteration point (for example, by a quadratic model through the sampled points). They are therefore less exposed to the relative error of the noise of the function because the sample points are never too close to each other to take the error into account. DFO might be useful even if the finite differences can be computed as the number of function evaluations is lower. This is particularly beneficial for problems where the evaluations of f are expensive. DFO solvers tend to exhibit a faster initial progress to the solution, however, they typically cannot achieve high-accurate solutions.

Minimization subject to bounds on the objective function

In all of the above problem categories it is assumed that
where a=- and b=+. Problems in which a and/or b are finite can be solved by adding an extra constraint of the appropriate type (i.e., linear or nonlinear) depending on the form of fx. Further advice is given in Section 3.7.

Multi-objective optimization

Sometimes a problem may have two or more objective functions which are to be optimized at the same time. Such problems are called multi-objective, multi-criteria or multi-attribute optimization. If the constraints are linear and the objectives are all linear then the terminology goal programming is also used.
Although there is no routine dealing with this type of problems explicitly in this mark of the Library, techniques used in this chapter and in Chapter E05 may be employed to address such problems, see Section 2.5.5.

Geometric Representation

To illustrate the nature of optimization problems it is useful to consider the following example in two dimensions:
fx = ex1 4x12 + 2x22 + 4 x1 x2+2 x2+1 .  
(This function is used as the example function in the documentation for the unconstrained routines.)
Figure 1
Figure 1
Figure 1 is a contour diagram of fx. The contours labelled F0 , F1 ,, F4  are isovalue contours, or lines along which the function fx takes specific constant values. The point x* = 12 , -1 T  is a local unconstrained minimum, that is, the value of fx* (=0) is less than at all the neighbouring points. A function may have several such minima. The point xs is said to be a saddle point because it is a minimum along the line AB, but a maximum along CD.
If we add the constraint x10 (a simple bound) to the problem of minimizing fx, the solution remains unaltered. In Figure 1 this constraint is represented by the straight line passing through x1=0, and the shading on the line indicates the unacceptable region (i.e., x1<0).
If we add the nonlinear constraint g1x : x1+ x2- x1 x2- 320 , represented by the curved shaded line in Figure 1, then x* is not a feasible point because g1x*<0. The solution of the new constrained problem is xb 1.1825,-1.7397T , the feasible point with the smallest function value (where fxb3.0607).

Sufficient Conditions for a Solution

All nonlinear functions will be assumed to have continuous second derivatives in the neighbourhood of the solution.

Unconstrained minimization

The following conditions are sufficient for the point x* to be an unconstrained local minimum of fx:
(i) fx*=0 and
(ii) 2fx* is positive definite,
where · denotes the Euclidean norm.

Minimization subject to bounds on the variables

At the solution of a bounds-constrained problem, variables which are not on their bounds are termed free variables. If it is known in advance which variables are on their bounds at the solution, the problem can be solved as an unconstrained problem in just the free variables; thus, the sufficient conditions for a solution are similar to those for the unconstrained case, applied only to the free variables.
Sufficient conditions for a feasible point x* to be the solution of a bounds-constrained problem are as follows:
(i) g-x*=0; and
(ii) G-x* is positive definite; and
(iii) xj fx*<0,xj=uj; xj fx*>0,xj=lj,
where g-x is the gradient of fx with respect to the free variables, and G-x is the Hessian matrix of fx with respect to the free variables. The extra condition (iii) ensures that fx cannot be reduced by moving off one or more of the bounds.

Linearly-constrained minimization

For the sake of simplicity, the following description does not include a specific treatment of bounds or range constraints, since the results for general linear inequality constraints can be applied directly to these cases.
At a solution x*, of a linearly-constrained problem, the constraints which hold as equalities are called the active or binding constraints. Assume that there are t active constraints at the solution x*, and let A^ denote the matrix whose columns are the columns of A corresponding to the active constraints, with b^ the vector similarly obtained from b; then
The matrix Z is defined as an n×n-t matrix satisfying:
A^TZ=0; ZTZ=I.  
The columns of Z form an orthogonal basis for the set of vectors orthogonal to the columns of A^.
At the solution of a linearly-constrained problem, the projected gradient vector must be zero, which implies that the gradient vector fx* can be written as a linear combination of the columns of A^, i.e., fx*=i=1tλi*a^i=A^λ*. The scalar λi* is defined as the Lagrange multiplier corresponding to the ith active constraint. A simple interpretation of the ith Lagrange multiplier is that it gives the gradient of fx along the ith active constraint normal; a convenient definition of the Lagrange multiplier vector (although not a recommended method for computation) is:
Sufficient conditions for x* to be the solution of a linearly-constrained problem are:
(i) x* is feasible, and A^Tx*=b^; and
(ii) gZx*=0, or equivalently, fx*=A^λ*; and
(iii) GZx* is positive definite; and
(iv) λi*>0 if λi* corresponds to a constraint a^iT x* b^i ;
λi*<0 if λi* corresponds to a constraint a^iT x* b^i .
The sign of λi* is immaterial for equality constraints, which by definition are always active.

Nonlinearly-constrained minimization

For nonlinearly-constrained problems, much of the terminology is defined exactly as in the linearly-constrained case. To simplify the notation, let us assume that all nonlinear constraints are in the form cx0. The set of active constraints at x again means the set of constraints that hold as equalities at x, with corresponding definitions of c^ and A^: the vector c^x contains the active constraint functions, and the columns of A^x are the gradient vectors of the active constraints. As before, Z is defined in terms of A^x as a matrix such that:
A^TZ=0; ZTZ=I  
where the dependence on x has been suppressed for compactness.
The projected gradient vector gZx is the vector ZTfx. At the solution x* of a nonlinearly-constrained problem, the projected gradient must be zero, which implies the existence of Lagrange multipliers corresponding to the active constraints, i.e., fx*=A^x*λ*.
The Lagrangian function is given by:
We define gLx as the gradient of the Lagrangian function; GLx as its Hessian matrix, and G^Lx as its projected Hessian matrix, i.e., G^L=ZTGLZ.
Sufficient conditions for x* to be the solution of a nonlinearly-constrained problem are:
(i) x* is feasible, and c^x*=0; and
(ii) gZx*=0, or, equivalently, fx*=A^x*λ*; and
(iii) G^Lx* is positive definite; and
(iv) λi*>0 if λi* corresponds to a constraint of the form c^i0.
The sign of λi* is immaterial for equality constraints, which by definition are always active.
Note that condition (ii) implies that the projected gradient of the Lagrangian function must also be zero at x*, since the application of ZT annihilates the matrix A^x*.

Background to Optimization Methods

All the algorithms contained in this chapter generate an iterative sequence x k  that converges to the solution x* in the limit, except for some special problem categories (i.e., linear and quadratic programming). To terminate computation of the sequence, a convergence test is performed to determine whether the current estimate of the solution is an adequate approximation. The convergence tests are discussed in Section 2.7.
Most of the methods construct a sequence x k  satisfying:
x k+1 =x k +α k p k ,  
where the vector p k  is termed the direction of search, and α k  is the steplength. The steplength α k  is chosen so that fx k+1 <fx k  and is computed using one of the techniques for one-dimensional optimization referred to in Section 2.5.1.

One-dimensional optimization

The Library contains two special routines for minimizing a function of a single variable. Both routines are based on safeguarded polynomial approximation. One routine requires function evaluations only and fits a quadratic polynomial whilst the other requires function and gradient evaluations and fits a cubic polynomial. See Section 4.1 of Gill et al. (1981).

Methods for unconstrained optimization

The distinctions between methods arise primarily from the need to use varying levels of information about derivatives of fx in defining the search direction. We describe three basic approaches to unconstrained problems, which may be extended to other problem categories. Since a full description of the methods would fill several volumes, the discussion here can do little more than allude to the processes involved, and direct you to other sources for a full explanation.
(a) Newton-type Methods (Modified Newton Methods)
Newton-type methods use the Hessian matrix 2fx k , or its finite difference approximation , to define the search direction. The routines in the Library either require a subroutine that computes the elements of the Hessian directly, or they approximate them by finite differences.
Newton-type methods are the most powerful methods available for general problems and will find the minimum of a quadratic function in one iteration. See Sections 4.4 and 4.5.1 of Gill et al. (1981).
(b) Quasi-Newton Methods
Quasi-Newton methods approximate the Hessian 2fxk by a matrix Bk which is modified at each iteration to include information obtained about the curvature of f along the current search direction pk. Although not as robust as Newton-type methods, quasi-Newton methods can be more efficient because the Hessian is not computed directly, or approximated by finite differences. Quasi-Newton methods minimize a quadratic function in n iterations, where n is the number of variables. See Section 4.5.2 of Gill et al. (1981).
(c) Conjugate-gradient Methods
Unlike Newton-type and quasi-Newton methods, conjugate-gradient methods do not require the storage of an n by n matrix and so are ideally suited to solve large problems. Conjugate-gradient type methods are not usually as reliable or efficient as Newton-type, or quasi-Newton methods. See Section 4.8.3 of Gill et al. (1981).

Methods for nonlinear least squares problems

These methods are similar to those for general nonlinear optimization, but exploit the special structure of the Hessian matrix to give improved computational efficiency.
the Hessian matrix is of the form
2fx = 2 JxT Jx + i=1m rix 2rix ,  
where Jx is the Jacobian matrix of rx.
In the neighbourhood of the solution, rx is often small compared to JxT Jx  (for example, when rx represents the goodness-of-fit of a nonlinear model to observed data). In such cases, 2 JxT Jx  may be an adequate approximation to 2fx, thereby avoiding the need to compute or approximate second derivatives of rix. See Section 4.7 of Gill et al. (1981).

Methods for handling constraints

Bounds on the variables are dealt with by fixing some of the variables on their bounds and adjusting the remaining free variables to minimize the function. By examining estimates of the Lagrange multipliers it is possible to adjust the set of variables fixed on their bounds so that eventually the bounds active at the solution should be correctly identified. This type of method is called an active-set method. One feature of such a method is that, given an initial feasible point, all approximations xk are feasible. This approach can be extended to general linear constraints. At a point, x, the set of constraints which hold as equalities being used to predict, or approximate, the set of active constraints is called the working set.
Nonlinear constraints are more difficult to handle. If at all possible, it is usually beneficial to avoid including nonlinear constraints during the formulation of the problem. The methods currently implemented in the Library handle nonlinearly constrained problems by transforming them into a sequence of quadratic programming problems. A feature of such methods is that xk is not guaranteed to be feasible except in the limit, and this is certainly true of the routines currently in the Library. See Chapter 6, particularly Sections 6.4 and 6.5, of Gill et al. (1981).
Anyone interested in a detailed description of methods for optimization should consult the references.

Methods for handling multi-objective optimization

Suppose we have objective functions fix, i>1, all of which we need to minimize at the same time. There are two main approaches to this problem:
(a) Combine the individual objectives into one composite objective. Typically this might be a weighted sum of the objectives, e.g.,
w1 f1x + w2 f2x + + wn fnx  
Here you choose the weights to express the relative importance of the corresponding objective. Ideally each of the fix should be of comparable size at a solution.
(b) Order the objectives in order of importance. Suppose fi are ordered such that fix is more important than fi+1x, for i=1,2,,n-1. Then in the lexicographical approach to multi-objective optimization a sequence of subproblems are solved. Firstly solve the problem for objective function f1x and denote by r1 the value of this minimum. If i-1 subproblems have been solved with results ri-1 then subproblem i becomes minfix subject to rkfkxrk, for k=1,2,,i-1 plus the other constraints.
Clearly the bounds on fk might be relaxed at your discretion.
In general, if NAG routines from the Chapter E04 are used then only local minima are found. This means that a better solution to an individual objective might be found without worsening the optimal solutions to the other objectives. Ideally you seek a Pareto solution; one in which an improvement in one objective can only be achieved by a worsening of another objective.
To obtain a Pareto solution routines from Chapter E05 might be used or, alternatively, a pragmatic attempt to derive a global minimum might be tried (see e05ucf). In this approach a variety of different minima are computed for each subproblem by starting from a range of different starting points. The best solution achieved is taken to be the global minimum. The more starting points chosen the greater confidence you might have in the computed global minimum.


Scaling (in a broadly defined sense) often has a significant influence on the performance of optimization methods.
Since convergence tolerances and other criteria are necessarily based on an implicit definition of ‘small’ and ‘large’, problems with unusual or unbalanced scaling may cause difficulties for some algorithms.
Although there are currently no user-callable scaling routines in the Library, scaling can be performed automatically in routines which solve sparse LP, QP or NLP problems and in some dense solver routines. Such routines have an optional parameter ‘Scale Option’ which can be set by the user; see individual routine documents for details.
The following sections present some general comments on problem scaling.

Transformation of variables

One method of scaling is to transform the variables from their original representation, which may reflect the physical nature of the problem, to variables that have certain desirable properties in terms of optimization. It is generally helpful for the following conditions to be satisfied:
(i) the variables are all of similar magnitude in the region of interest;
(ii) a fixed change in any of the variables results in similar changes in fx. Ideally, a unit change in any variable produces a unit change in fx;
(iii) the variables are transformed so as to avoid cancellation error in the evaluation of fx.
Normally, you should restrict yourself to linear transformations of variables, although occasionally nonlinear transformations are possible. The most common such transformation (and often the most appropriate) is of the form
where D is a diagonal matrix with constant coefficients. Our experience suggests that more use should be made of the transformation
where v is a constant vector.
Consider, for example, a problem in which the variable x3 represents the position of the peak of a Gaussian curve to be fitted to data for which the extreme values are 150 and 170; therefore x3 is known to lie in the range 150170. One possible scaling would be to define a new variable x-3, given by
A better transformation, however, is given by defining x-3 as
Frequently, an improvement in the accuracy of evaluation of fx can result if the variables are scaled before the routines to evaluate fx are coded. For instance, in the above problem just mentioned of Gaussian curve-fitting, x3 may always occur in terms of the form x3-xm, where xm is a constant representing the mean peak position.

Scaling the objective function

The objective function has already been mentioned in the discussion of scaling the variables. The solution of a given problem is unaltered if fx is multiplied by a positive constant, or if a constant value is added to fx. It is generally preferable for the objective function to be of the order of unity in the region of interest; thus, if in the original formulation fx is always of the order of 10+5 (say), then the value of fx should be multiplied by 10-5 when evaluating the function within an optimization routine. If a constant is added or subtracted in the computation of fx, usually it should be omitted, i.e., it is better to formulate fx as x12+x22 rather than as x12+x22+1000 or even x12+x22+1. The inclusion of such a constant in the calculation of fx can result in a loss of significant figures.

Scaling the constraints

A ‘well scaled’ set of constraints has two main properties. Firstly, each constraint should be well-conditioned with respect to perturbations of the variables. Secondly, the constraints should be balanced with respect to each other, i.e., all the constraints should have ‘equal weight’ in the solution process.
The solution of a linearly- or nonlinearly-constrained problem is unaltered if the ith constraint is multiplied by a positive weight wi. At the approximation of the solution determined by an active-set solver, any active linear constraints will (in general) be satisfied ‘exactly’ (i.e., to within the tolerance defined by machine precision) if they have been properly scaled. This is in contrast to any active nonlinear constraints, which will not (in general) be satisfied ‘exactly’ but will have ‘small’ values (for example, g^1x*=10-8, g^2x*=-10 -6, and so on). In general, this discrepancy will be minimized if the constraints are weighted so that a unit change in x produces a similar change in each constraint.
A second reason for introducing weights is related to the effect of the size of the constraints on the Lagrange multiplier estimates and, consequently, on the active-set strategy. This means that different sets of weights may cause an algorithm to produce different sequences of iterates. Additional discussion is given in Gill et al. (1981).

Analysis of Computed Results

Convergence criteria

The convergence criteria inevitably vary from routine to routine, since in some cases more information is available to be checked (for example, is the Hessian matrix positive definite?), and different checks need to be made for different problem categories (for example, in constrained minimization it is necessary to verify whether a trial solution is feasible). Nonetheless, the underlying principles of the various criteria are the same; in non-mathematical terms, they are:
(i) is the sequence x k  converging?
(ii) is the sequence f k  converging?
(iii) are the necessary and sufficient conditions for the solution satisfied?
The decision as to whether a sequence is converging is necessarily speculative. The criterion used in the present routines is to assume convergence if the relative change occurring between two successive iterations is less than some prescribed quantity. Criterion (iii) is the most reliable but often the conditions cannot be checked fully because not all the required information may be available.

Checking results

Little a priori guidance can be given as to the quality of the solution found by a nonlinear optimization algorithm, since no guarantees can be given that the methods will not fail. Therefore, you should always check the computed solution even if the routine reports success. Frequently a ‘solution’ may have been found even when the routine does not report a success. The reason for this apparent contradiction is that the routine needs to assess the accuracy of the solution. This assessment is not an exact process and consequently may be unduly pessimistic. Any ‘solution’ is in general only an approximation to the exact solution, and it is possible that the accuracy you have specified is too stringent.
Further confirmation can be sought by trying to check whether or not convergence tests are almost satisfied, or whether or not some of the sufficient conditions are nearly satisfied. When it is thought that a routine has returned a nonzero value of ifail only because the requirements for ‘success’ were too stringent it may be worth restarting with increased convergence tolerances.
For constrained problems, check whether the solution returned is feasible, or nearly feasible; if not, the solution returned is not an adequate solution.
Confidence in a solution may be increased by restarting the solver with a different initial approximation to the solution. See Section 8.3 of Gill et al. (1981) for further information.

Monitoring progress

Many of the routines in the chapter have facilities to allow you to monitor the progress of the minimization process, and you are encouraged to make use of these facilities. Monitoring information can be a great aid in assessing whether or not a satisfactory solution has been obtained, and in indicating difficulties in the minimization problem or in the ability of the routine to cope with the problem.
The behaviour of the function, the estimated solution and first derivatives can help in deciding whether a solution is acceptable and what to do in the event of a return with a nonzero value of ifail.

Confidence intervals for least squares solutions

When estimates of the parameters in a nonlinear least squares problem have been found, it may be necessary to estimate the variances of the parameters and the fitted function. These can be calculated from the Hessian of the objective fx at the solution.
In many least squares problems, the Hessian is adequately approximated at the solution by G=2JTJ (see Section 2.5.3). The Jacobian, J, or a factorization of J is returned by all the comprehensive least squares routines and, in addition, a routine is available in the Library to estimate variances of the parameters following the use of most of the nonlinear least squares routines, in the case that G=2JTJ is an adequate approximation.
Let H be the inverse of G, and S be the sum of squares, both calculated at the solution x-; an unbiased estimate of the variance of the ith parameter xi is
varx-i=2S m-n Hii  
and an unbiased estimate of the covariance of x-i and x-j is
covarx-i,x-j=2S m-n Hij.  
If x* is the true solution, then the 1001-β% confidence interval on x- is
x-i-varx-i. t1-β/2,m-n<xi*<x-i+varx-i.t1-β/2,m-n,  i=1,2,,n  
where t1-β/2,m-n is the 1001-β/2 percentage point of the t-distribution with m-n degrees of freedom.
In the majority of problems, the residuals ri, for i=1,2,,m, contain the difference between the values of a model function ϕz,x calculated for m different values of the independent variable z, and the corresponding observed values at these points. The minimization process determines the parameters, or constants x, of the fitted function ϕz,x. For any value, z-, of the independent variable z, an unbiased estimate of the variance of ϕ is
varϕ=2S m-n i=1n j=1n ϕ xi z- ϕ xj z- Hij.  
The 1001-β% confidence interval on f at the point z- is
For further details on the analysis of least squares solutions see Bard (1974) and Wolberg (1967).

Recommendations on Choice and Use of Available Routines

The choice of routine depends on several factors: the type of problem (unconstrained, etc.); the level of derivative information available (function values only, etc.); your experience (there are easy-to-use versions of some routines); whether or not a problem is sparse; whether or not the routine is to be used in a multithreaded environment; and whether computational time has a high priority. Not all choices are catered for in the current version of the Library.

Easy-to-use and Comprehensive Routines

Many routines appear in the Library in two forms: a comprehensive form and an easy-to-use form. The purpose of the easy-to-use forms is to make the routine simple to use by including in the calling sequence only those arguments absolutely essential to the definition of the problem, as opposed to arguments relevant to the solution method. If you are an experienced user the comprehensive routines have additional arguments which enable you to improve their efficiency by ‘tuning’ the method to a particular problem. If you are a casual or inexperienced user, this feature is of little value and may in some cases cause a failure because of a poor choice of some arguments.
In the easy-to-use routines, these extra arguments are determined either by fixing them at a known safe and reasonably efficient value, or by an auxiliary routine which generates a ‘good’ value automatically.
For routines introduced since Mark 12 of the Library a different approach has been adopted towards the choice of easy-to-use and comprehensive routines. The optimization routine has an easy-to-use argument list, but additional arguments may be changed from their default values by calling an ‘option’ setting routine before the call to the main optimization routine. This approach has the advantages of allowing the options to be given in the form of keywords and requiring only those options that are to be different from their default values to be set.

Thread Safe Routines

Many of the routines in this chapter come in pairs, with each routine in the pair having exactly the same functionality, except that one of them has additional arguments in order to make it safe for use in multithreaded applications. The routine that is safe for use in multithreaded applications has an ‘a’ as the last character in the name, in place of the usual ‘f’.
An example of such a pair is e04aba and e04abf.

Reverse Communication Routines

Most of the routines in this chapter are called just once in order to compute the minimum of a given objective function subject to a set of constraints on the variables. The objective function and nonlinear constraints (if any) are specified by you and written as subroutines to a very rigid format described in the relevant routine document.
This chapter also contains a pair of reverse communication routines, e04uff/e04ufa, which solve dense NLP problems using a sequential quadratic programming method. These may be convenient to use when the minimization routine is being called from a computer language which does not fully support procedure arguments in a way that is compatible with the Library. These routines are also useful if a large amount of data needs to be transmitted into the routine. See Section 3.3.3 in How to Use the NAG Library and its Documentation for more information about reverse communication routines.

Choosing Between Variant Routines for Some Problems

As evidenced by the wide variety of routines available in Chapter E04, it is clear that no single algorithm can solve all optimization problems. It is important to try to match the problem to the most suitable routine, and that is what the decision trees in Section 4 help to do.
Sometimes in Chapter E04 more than one routine is available to solve precisely the same minimization problem. Thus, for example, the general nonlinear programming routines e04ucf/e04uca and e04wdf are based on similar methods. Experience shows that although both routines can usually solve the same problem and get similar results, sometimes one routine will be faster, sometimes one might find a different local minimum to the other, or, in difficult cases, one routine may obtain a solution when the other one fails.
After using one of these routines, if the results obtained are unacceptable for some reason, it may be worthwhile trying the other routine instead. In the absence of any other information, in the first instance you are recommended to try using e04ucf/e04uca, and if that proves unsatisfactory, try using e04wdf. Although the algorithms used are very similar, the two routines each have slightly different optional parameters which may allow the course of the computation to be altered in different ways.
Other pairs of routines which solve the same kind of problem are e04nqf (recommended first choice) or e04nkf/e04nka, for sparse quadratic or linear programming problems, and e04vhf (recommended) or e04ugf/e04uga, for sparse nonlinear programming. In these cases the argument lists are not so similar as e04ucf/e04uca or e04wdf, but the same considerations apply.

NAG Optimization Modelling Suite

Mark 26 of the Library introduced NAG optimization modelling suite, a suite of routines which allows you to define and solve various optimization problems in a uniform manner. The first key feature of the suite is that the definition of the optimization problem and the call to the solver have been separated so it is possible to set up a problem in the same way for different solvers. The second feature is that the problem representation is built up from basic components (for example, a QP problem is composed of a quadratic objective, simple bounds and linear constraints), therefore different types of problems reuse the same routines for their common parts.
A connecting element to all routines in the suite is a handle, a pointer to an internal data structure, which is passed among the routines. It holds all information about the problem, the solution and the solver. Each handle should go through four stages in its life: initialization, problem formulation, problem solution and deallocation.
The initialization is performed by e04raf which creates an empty problem with n decision variables. A call to e04rzf marks the end of the life of the handle as it deallocates all the allocated memory and data within the handle and destroys the handle itself. After the initialization, the objective may be defined as one of the following: The routines for constraint definition are
These routines may be called in an arbitrary order, however, a call to e04rnf must precede a call to e04rpf for the matrix inequalities with bilinear terms and the nonlinear objective or constraints (e04rgf or e04rkf) must precede the definition of the second derivatives by e04rlf. For further details please refer to the documentation of the individual routines.
The suite also includes the following service routines:
When the problem is fully formulated, the handle can be passed to a solver which is compatible with the defined problem. At the current mark of the Library the NAG optimization modelling suite comprises of e04fff, e04mtf, e04stf and e04svf. The solver indicates by an error flag if it cannot deal with the given formulation. A diagram of the life cycle of the handle is depicted in Figure 2.
Figure figureoptsuite
Figure 2

Service Routines

One of the most common errors in the use of optimization routines is that user-supplied subroutines do not evaluate the relevant partial derivatives correctly. Because exact gradient information normally enhances efficiency in all areas of optimization, you are encouraged to provide analytical derivatives whenever possible. However, mistakes in the computation of derivatives can result in serious and obscure run-time errors. Consequently, service routines are provided to perform an elementary check on the gradients you supplied. These routines are inexpensive to use in terms of the number of calls they require to user-supplied subroutines.
The appropriate checking routines are as follows:
Minimization routine Checking routine(s)
e04kdf e04hcf
e04lbf e04hcf and e04hdf
e04gbf e04yaf
e04gdf e04yaf
e04hef e04yaf and e04ybf
It should be noted that routines e04stf, e04ucf/e04uca, e04uff/e04ufa, e04ugf/e04uga, e04usf/e04usa, e04vhf and e04wdf each incorporate a check on the derivatives being supplied. This involves verifying the gradients at the first point that satisfies the linear constraints and bounds. There is also an option to perform a more reliable (but more expensive) check on the individual gradient elements being supplied. Note that the checks are not infallible.
A second type of service routine computes a set of finite differences to be used when approximating first derivatives. Such differences are required as input arguments by some routines that use only function evaluations.
e04ycf estimates selected elements of the variance-covariance matrix for the computed regression parameters following the use of a nonlinear least squares routine.
e04xaf/e04xaa estimates the gradient and Hessian of a function at a point, given a routine to calculate function values only, or estimates the Hessian of a function at a point, given a routine to calculate function and gradient values.

Function Evaluations at Infeasible Points

All the solvers for constrained problems based on active-set method will ensure that any evaluations of the objective function occur at points which approximately (up to the given tolerance) satisfy any simple bounds or linear constraints.
There is no attempt to ensure that the current iteration satisfies any nonlinear constraints. If you wish to prevent your objective function being evaluated outside some known region (where it may be undefined or not practically computable), you may try to confine the iteration within this region by imposing suitable simple bounds or linear constraints (but beware as this may create new local minima where these constraints are active).
Note also that some routines allow you to return the argument (iflag, inform, mode or status) with a negative value to indicate when the objective function (or nonlinear constraints where appropriate) cannot be evaluated. In case the routine cannot recover (e.g., cannot find a different trial point), it forces an immediate clean exit from the routine.

Related Problems

Apart from the standard types of optimization problem, there are other related problems which can be solved by routines in this or other chapters of the Library.
h02bbf solves dense integer LP problems, h02cbf solves dense integer QP problems, h02cef solves sparse integer QP problems, h02daf solves dense mixed integer NLP problems and h03abf solves a special type of such problem known as a ‘transportation’ problem.
Several routines in Chapters F04 and F08 solve linear least squares problems, i.e., minimizei=1mri x 2 where rix=bi-j=1naijxj.
e02gaf solves an overdetermined system of linear equations in the l1 norm, i.e., minimizes i=1mrix, with ri as above, and e02gbf solves the same problem subject to linear inequality constraints.
e02gcf solves an overdetermined system of linear equations in the l norm, i.e., minimizes maxirix, with ri as above.
Chapter E05 contains routines for global minimization.
Section 2.5.5 describes how a multi-objective optimization problem might be addressed using routines from this chapter and from Chapter E05.

Decision Trees

  no objective linear quadratic nonlinear sum of squares
unconstrained     QP
See Tree 2
See Tree 3
See Tree 4
simple bounds LP
See Tree 1
See Tree 1
See Tree 2
See Tree 3
See Tree 4
linear LP
See Tree 1
See Tree 1
See Tree 2
See Tree 3
See Tree 4
nonlinear NLP
See Tree 3
See Tree 3
See Tree 3
See Tree 3
See Tree 4
matrix inequalities e04svf e04svf e04svf    
Table 1
Decision Matrix

Tree 1: Linear Programming (LP)

Is the problem sparse/large-scale?   e04mtf, e04nqf, e04nkf
e04mff, e04ncf

Tree 2: Quadratic Programming (QP)

Is the problem sparse/large-scale?   Is it convex?   e04nqf, e04stf, e04nkf
  no   no
e04stf, e04vhf, e04ugf
Is it convex?   e04ncf

Tree 3: Nonlinear Programming (NLP)

Is the problem sparse/large-scale?   Is it unconstrained?   Are first derivatives available?   e04stf, e04dga, e04vhf, e04uga
  no   no   no
e04vhf, e04uga
Are first derivatives available?   Are second derivatives available?   e04stf
  no   no
e04vhf, e04stf, e04uga
e04vhf, e04uga
Are there linear or nonlinear constraints?   e04uca, e04ufa, e04wdf
Is there only one variable?   Are first derivatives available?   e04bba
  no   no
Is it unconstrained with the objective with many discontinuities?   e04cbf or e05saf
Are first derivatives available?   Are second derivatives available?   Are you an experienced user?   e04lbf
  no   no   no
Are many function evaluations problematic?   Are you an experienced user?   e04uca, e04ufa, e04wdf
  no   no
Are you an experienced user?   e04kdf
Is the objective expensive to evaluate or noisy?   e04jcf
Are you an experienced user?   e04uca, e04ufa, e04wdf

Tree 4: Least squares problems (LSQ)

Is the objective sum of squared linear functions and no nonlinear constraints?   Are there linear constraints?   e04ncf
  no   no
Are there simple bounds?   e04pcf, e04ncf
Chapters F04, F07 or F08 or e04pcf, e04ncf
Are there linear or nonlinear constraints?   e04usf
Are there bound constraints?   Are first derivatives available?   e04usf
  no   no
Are first derivatives available?   Are second derivatives available?   Are you an experienced user?   e04hef
  no   no   no
Are many function evaluations problematic?   Are you an experienced user?   e04gbf
  no   no
Are you an experienced user?   e04gdf
e04fff, e04fcf

Functionality Index

Linear programming (LP), 
        active-set method/primal simplex, 
            alternative 1 e04mff
            alternative 2 e04ncf
        interior point method (IPM) e04mtf
        active-set method/primal simplex, 
            recommended (see Section 3.4) e04nqf
            alternative e04nkf
Quadratic programming (QP), 
        active-set method for (possibly nonconvex) QP problem e04nff
        active-set method for convex QP problem e04ncf
        active-set method sparse convex QP problem, 
            recommended (see Section 3.4) e04nqf
            alternative e04nkf
            interior point method (IPM) for (possibly nonconvex) QP problems e04stf
Nonlinear programming (NLP), 
        active-set sequential quadratic programming (SQP), 
            recommended (see Section 3.4) e04ucf
            alternative e04wdf
            reverse communication e04uff
        interior point method (IPM) e04stf
        active-set sequential quadratic programming (SQP), 
            recommended (see Section 3.4) e04vhf
            alternative e04ugf
Nonlinear programming (NLP) – derivative-free optimization (DFO), 
    model-based method for bound-constrained optimization e04jcf
    Nelder–Mead simplex method for unconstrained optimization e04cbf
Nonlinear programming (NLP) – special cases, 
    unidimensional optimization (one-dimensional) with bound constraints, 
        method based on quadratic interpolation, no derivatives e04abf
        method based on cubic interpolation e04bbf
        preconditioned conjugate gradient method e04dgf
        quasi-Newton algorithm, no derivatives e04jyf
        quasi-Newton algorithm, first derivatives e04kyf
        modified Newton algorithm, first derivatives e04kdf
        modified Newton algorithm, first derivatives, easy-to-use e04kzf
        modified Newton algorithm, first and second derivatives e04lbf
        modified Newton algorithm, first and second derivatives, easy-to-use e04lyf
Semidefinite programming (SDP), 
    generalized augmented Lagrangian method for SDP and SDP with bilinear matrix inequalities (BMI-SDP) e04svf
Linear least squares, linear regression, data fitting, 
        bound-constrained least squares problem e04pcf
        linearly-constrained active-set method e04ncf
Nonlinear least squares, data fitting, 
        combined Gauss–Newton and modified Newton algorithm, 
            no derivatives e04fcf
            no derivatives, easy-to-use e04fyf
            first derivatives e04gdf
            first derivatives, easy-to-use e04gzf
            first and second derivatives e04hef
            first and second derivatives, easy-to-use e04hyf
        combined Gauss–Newton and quasi-Newton algorithm, 
            first derivatives e04gbf
            first derivatives, easy-to-use e04gyf
        covariance matrix for nonlinear least squares problem (unconstrained) e04ycf
        model-based derivative-free algorithm e04fff
    bound constrained, 
        nonlinear constraints active-set sequential quadratic programming (SQP) e04usf
        model-based derivative-free algorithm e04fff
NAG optimization modelling suite, 
    initialization of a handle for the NAG optimization modelling suite e04raf
    define a linear objective function e04ref
    define a linear or a quadratic objective function e04rff
    define a nonlinear least square objective function e04rmf
    define a nonlinear objective function e04rgf
    define bounds of variables e04rhf
    define a block of linear constraints e04rjf
    define a block of nonlinear constraints e04rkf
    define a structure of Hessian of the objective, constraints or the Lagrangian e04rlf
    add one or more linear matrix inequality constraints e04rnf
    define bilinear matrix terms e04rpf
    print information about a problem handle e04ryf
    set/get information in a problem handle e04rxf
    destroy the problem handle e04rzf
    interior point method (IPM) for linear programming    (LP) e04mtf
    interior point method (IPM) for nonlinear programming (NLP) e04stf
    generalized augmented Lagrangian method for SDP and SDP with bilinear matrix inequalities (BMI-SDP) e04svf
    supply optional parameter values from a character string e04zmf
    get the setting of option e04znf
    supply optional parameter values from external file e04zpf
Service routines, 
    input and output (I/O), 
        read MPS data file defining LP, QP, MILP or MIQP problem e04mxf
        write MPS data file defining LP, QP, MILP or MIQP problem e04mwf
        read sparse SPDA data files for linear SDP problems e04rdf
        read MPS data file defining LP or QP problem (deprecated) e04mzf
    derivative check and approximation, 
        check user's routine for calculating first derivatives of function e04hcf
        check user's routine for calculating second derivatives of function e04hdf
        check user's routine for calculating Jacobian of first derivatives e04yaf
        check user's routine for calculating Hessian of a sum of squares e04ybf
        estimate (using numerical differentiation) gradient and/or Hessian of a function e04xaf
        determine the pattern of nonzeros in the Jacobian matrix for e04vhf e04vjf
    covariance matrix for nonlinear least squares problem (unconstrained) e04ycf
    option setting routines, 
        NAG optimization modelling suite, 
            supply optional parameter values from a character string e04zmf
            get the setting of option e04znf
            supply optional parameter values from external file e04zpf
            initialization routine for e04dga e04wbf
            supply optional parameter values from external file e04djf
            supply optional parameter values from a character string e04dkf
            initialization routine for e04mfa e04wbf
            supply optional parameter values from external file e04mgf
            supply optional parameter values from a character string e04mhf
            initialization routine for e04nca e04wbf
            supply optional parameter values from external file e04ndf
            supply optional parameter values from a character string e04nef
            initialization routine for e04nfa e04wbf
            supply optional parameter values from external file e04ngf
            supply optional parameter values from a character string e04nhf
            initialization routine for e04nka e04wbf
            supply optional parameter values from external file e04nlf
            supply optional parameter values from a character string e04nmf
            initialization routine e04npf
            supply optional parameter values from external file e04nrf
            set a single option from a character string e04nsf
            set a single option from an integer argument e04ntf
            set a single option from a real argument e04nuf
            get the setting of an integer valued option e04nxf
            get the setting of a real valued option e04nyf
        e04ucf/e04uca and e04uff/e04ufa, 
            initialization routine for e04uca and e04ufa e04wbf
            supply optional parameter values from external file e04udf
            supply optional parameter values from a character string e04uef
            initialization routine for e04uga e04wbf
            supply optional parameter values from external file e04uhf
            supply optional parameter values from a character string e04ujf
            initialization routine for e04usa e04wbf
            supply optional parameter values from external file e04uqf
            supply optional parameter values from a character string e04urf
            initialization routine e04vgf
            supply optional parameter values from external file e04vkf
            set a single option from a character string e04vlf
            set a single option from an integer argument e04vmf
            set a single option from a real argument e04vnf
            get the setting of an integer valued option e04vrf
            get the setting of a real valued option e04vsf
            initialization routine e04wcf
            supply optional parameter values from external file e04wef
            set a single option from a character string e04wff
            set a single option from an integer argument e04wgf
            set a single option from a real argument e04whf
            get the setting of an integer valued option e04wkf
            get the setting of a real valued option e04wlf

Auxiliary Routines Associated with Library Routine Arguments

See the description of the argument monit in e04cbf.
See the description of the argument lsqlin in e04gbf.
See the description of the argument lsqmon in e04fcf, e04gdf and e04hef.
See the description of the argument mon in e04fff.
See the description of the argument lsqlin in e04gbf.
See the description of the argument monfun in e04jcf.
See the description of the argument monit in e04mtf.
See the description of the argument qphess in e04nff/e04nfa and h02cbf.
See the description of the argument qphess in e04nff/e04nfa and h02cbf.
See the description of the argument qphx in e04nkf/e04nka and h02cef.
See the description of the argument qphx in e04nkf/e04nka and h02cef.
See the description of the argument qphx in e04nqf.
See the description of the argument mon in e04stf.
See the description of the argument objfun in e04stf.
See the description of the argument objgrd in e04stf.
See the description of the argument confun in e04stf.
See the description of the argument congrd in e04stf.
See the description of the argument hess in e04stf.
See the description of the argument confun in e04ucf/e04uca and e04usf/e04usa.
See the description of the argument confun in e04ugf/e04uga.
See the description of the argument objfun in e04ugf/e04uga.
See the description of the argument confun in e04wdf.

Routines Withdrawn or Scheduled for Withdrawal

The following lists all those routines that have been withdrawn since Mark 19 of the Library or are scheduled for withdrawal at one of the next two marks.
Mark of

Replacement Routine(s)
e04zcf/e04zca24No longer required


Bard Y (1974) Nonlinear Parameter Estimation Academic Press
Chvátal V (1983) Linear Programming W.H. Freeman
Dantzig G B (1963) Linear Programming and Extensions Princeton University Press
Fletcher R (1987) Practical Methods of Optimization (2nd Edition) Wiley
Gill P E and Murray W (ed.) (1974) Numerical Methods for Constrained Optimization Academic Press
Gill P E, Murray W and Wright M H (1981) Practical Optimization Academic Press
Murray W (ed.) (1972) Numerical Methods for Unconstrained Optimization Academic Press
Nocedal J and Wright S J (2006) Numerical Optimization (2nd Edition) Springer Series in Operations Research, Springer, New York
Wolberg J R (1967) Prediction Analysis Van Nostrand