Integer, Intent (In)	::	ldg, n, k, maxit, ldx
Integer, Intent (Inout)	::	ifail
Integer, Intent (Out)	::	iter, feval
Real (Kind=nag_wp), Intent (In)	::	errtol
Real (Kind=nag_wp), Intent (Inout)	::	g(ldg,), x(ldx,)
Real (Kind=nag_wp), Intent (Out)	::	nrmpgd

C Header Interface

#include <nag.h>

void	g02aef_ (double g[], const Integer ldg, const Integer n, const Integer k, const double errtol, const Integer maxit, double x[], const Integer ldx, Integer iter, Integer feval, double nrmpgd, Integer ifail)

The routine may be called by the names g02aef or nagf_correg_corrmat_nearest_kfactor.

3 Description

A correlation matrix

C

with

k

-factor structure may be characterised as a real square matrix that is symmetric, has a unit diagonal, is positive semidefinite and can be written as

C = X X^{T} + diag (I - X X^{T})

, where

I

is the identity matrix and

X

has

n

rows and

k

columns.

X

is often referred to as the factor loading matrix.

g02aef applies a spectral projected gradient method to the modified problem

{\min ‖G - X X^{T} + diag (X X^{T} - I)‖}_{F}

such that

{‖x_{i}^{T}‖}_{2} \leq 1

, for

i = 1, 2, \dots, n

, where

x_{i}

is the

i

th row of the factor loading matrix,

X

, which gives us the solution.

4 References

Birgin E G, Martínez J M and Raydan M (2001) Algorithm 813: SPG–software for convex-constrained optimization ACM Trans. Math. Software 27 340–349

Borsdorf R, Higham N J and Raydan M (2010) Computing a nearest correlation matrix with factor structure SIAM J. Matrix Anal. Appl. 31(5) 2603–2622

5 Arguments

1: $g (ldg *)$ – Real (Kind=nag_wp) array Input/Output: Note: the second dimension of the array g must be at least $n$ .

On entry: $G$ , the initial matrix.

On exit: a symmetric matrix $\frac{1}{2} (G + G^{T})$ with the diagonal elements set to unity.
2: $ldg$ – Integer Input: On entry: the first dimension of the array g as declared in the (sub)program from which g02aef is called.

Constraint: $ldg \geq n$ .
3: $n$ – Integer Input: On entry: $n$ , the order of the matrix $G$ .

Constraint: $n > 0$ .
4: $k$ – Integer Input: On entry: $k$ , the number of factors and columns of $X$ .

Constraint: $0 < k \leq n$ .
5: $errtol$ – Real (Kind=nag_wp) Input: On entry: the termination tolerance for the projected gradient norm. See references for further details. If $errtol \leq 0.0$ , $0.01$ is used. This is often a suitable default value.
6: $maxit$ – Integer Input: On entry: specifies the maximum number of iterations in the spectral projected gradient method.
If $maxit \leq 0$ , $40000$ is used.
7: $x (ldx *)$ – Real (Kind=nag_wp) array Output: Note: the second dimension of the array x must be at least $k$ .

On exit: contains the matrix $X$ .
8: $ldx$ – Integer Input: On entry: the first dimension of the array x as declared in the (sub)program from which g02aef is called.

Constraint: $ldx \geq n$ .
9: $iter$ – Integer Output: On exit: the number of steps taken in the spectral projected gradient method.
10: $feval$ – Integer Output: On exit: the number of evaluations of ${‖G - X X^{T} + diag (X X^{T} - I)‖}_{F}$ .
11: $nrmpgd$ – Real (Kind=nag_wp) Output: On exit: the norm of the projected gradient at the final iteration.
12: $ifail$ – Integer Input/Output: On entry: ifail must be set to $0$ , $- 1 or 1$ . If you are unfamiliar with this argument you should refer to Section 4 in the Introduction to the NAG Library FL Interface for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value $- 1 or 1$ is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this argument, the recommended value is $0$ . When the value $- 1 or 1$ is used it is essential to test the value of ifail on exit.

On exit: $ifail = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

- 1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $k = 〈value〉$ and $n = 〈value〉$ .
Constraint: $0 < k \leq n$ .

On entry, $ldg = 〈value〉$ and $n = 〈value〉$ .
Constraint: $ldg \geq n$ .

On entry, $ldx = 〈value〉$ and $n = 〈value〉$ .
Constraint: $ldx \geq n$ .

On entry, $n = 〈value〉$ .
Constraint: $n > 0$ .

$ifail = 2$: Spectral gradient method fails to converge in $〈value〉$ iterations.

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

The returned accuracy is controlled by errtol and limited by machine precision.

8 Parallelism and Performance

g02aef is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

g02aef makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.

Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

Arrays are internally allocated by g02aef. The total size of these arrays is

n \times n + 4 \times n \times k + (nb + 3) \times n + n + 50

real elements and

6 \times n

integer elements. Here

nb

is the block size required for optimal performance by f08fef and f08fgf which are called internally. All allocated memory is freed before return of g02aef.

See g03caf for constructing the factor loading matrix from a known correlation matrix.

10 Example

This example finds the nearest correlation matrix with

k = 2

factor structure to:

G = (\begin{matrix} 2 & - 1 & 0 & 0 \\ - 1 & 2 & - 1 & 0 \\ 0 & - 1 & 2 & - 1 \\ 0 & 0 & - 1 & 2 \end{matrix})

Interfaces: FL CL AD

NAG FL Interface Introduction

G02 (Correg) Chapter Contents

G02 (Correg) Chapter Introduction

g02ae: FL CL AD

NAG FL Interfaceg02aef (corrmat_​nearest_​kfactor)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
g02aef (corrmat_nearest_kfactor)