NAG Library Routine Document

f04bjf (real_symm_packed_solve)


    1  Purpose
    7  Accuracy


f04bjf computes the solution to a real system of linear equations AX=B, where A is an n by n symmetric matrix, stored in packed format and X and B are n by r matrices. An estimate of the condition number of A and an error bound for the computed solution are also returned.


Fortran Interface
Subroutine f04bjf ( uplo, n, nrhs, ap, ipiv, b, ldb, rcond, errbnd, ifail)
Integer, Intent (In):: n, nrhs, ldb
Integer, Intent (Inout):: ifail
Integer, Intent (Out):: ipiv(n)
Real (Kind=nag_wp), Intent (Inout):: ap(*), b(ldb,*)
Real (Kind=nag_wp), Intent (Out):: rcond, errbnd
Character (1), Intent (In):: uplo
C Header Interface
#include nagmk26.h
void  f04bjf_ (const char *uplo, const Integer *n, const Integer *nrhs, double ap[], Integer ipiv[], double b[], const Integer *ldb, double *rcond, double *errbnd, Integer *ifail, const Charlen length_uplo)


The diagonal pivoting method is used to factor A as A=UDUT, if uplo='U', or A=LDLT, if uplo='L', where U (or L) is a product of permutation and unit upper (lower) triangular matrices, and D is symmetric and block diagonal with 1 by 1 and 2 by 2 diagonal blocks. The factored form of A is then used to solve the system of equations AX=B.


Anderson E, Bai Z, Bischof C, Blackford S, Demmel J, Dongarra J J, Du Croz J J, Greenbaum A, Hammarling S, McKenney A and Sorensen D (1999) LAPACK Users' Guide (3rd Edition) SIAM, Philadelphia
Higham N J (2002) Accuracy and Stability of Numerical Algorithms (2nd Edition) SIAM, Philadelphia


1:     uplo – Character(1)Input
On entry: if uplo='U', the upper triangle of the matrix A is stored.
If uplo='L', the lower triangle of the matrix A is stored.
Constraint: uplo='U' or 'L'.
2:     n – IntegerInput
On entry: the number of linear equations n, i.e., the order of the matrix A.
Constraint: n0.
3:     nrhs – IntegerInput
On entry: the number of right-hand sides r, i.e., the number of columns of the matrix B.
Constraint: nrhs0.
4:     ap* – Real (Kind=nag_wp) arrayInput/Output
Note: the dimension of the array ap must be at least max1,n×n+1/2.
On entry: the n by n symmetric matrix A, packed column-wise in a linear array. The jth column of the matrix A is stored in the array ap as follows:
More precisely,
  • if uplo='U', the upper triangle of A must be stored with element Aij in api+jj-1/2 for ij;
  • if uplo='L', the lower triangle of A must be stored with element Aij in api+2n-jj-1/2 for ij.
On exit: if ifail0, the block diagonal matrix D and the multipliers used to obtain the factor U or L from the factorization A=UDUT or A=LDLT as computed by f07pdf (dsptrf), stored as a packed triangular matrix in the same storage format as A.
5:     ipivn – Integer arrayOutput
On exit: if ifail0, details of the interchanges and the block structure of D, as determined by f07pdf (dsptrf).
  • If ipivk>0, then rows and columns k and ipivk were interchanged, and dkk is a 1 by 1 diagonal block;
  • if uplo='U' and ipivk=ipivk-1<0, then rows and columns k-1 and -ipivk were interchanged and dk-1:k,k-1:k is a 2 by 2 diagonal block;
  • if uplo='L' and ipivk=ipivk+1<0, then rows and columns k+1 and -ipivk were interchanged and dk:k+1,k:k+1 is a 2 by 2 diagonal block.
6:     bldb* – Real (Kind=nag_wp) arrayInput/Output
Note: the second dimension of the array b must be at least max1,nrhs.
On entry: the n by r matrix of right-hand sides B.
On exit: if ifail=0 or n+1, the n by r solution matrix X.
7:     ldb – IntegerInput
On entry: the first dimension of the array b as declared in the (sub)program from which f04bjf is called.
Constraint: ldbmax1,n.
8:     rcond – Real (Kind=nag_wp)Output
On exit: if no constraints are violated, an estimate of the reciprocal of the condition number of the matrix A, computed as rcond=1/A1A-11.
9:     errbnd – Real (Kind=nag_wp)Output
On exit: if ifail=0 or n+1, an estimate of the forward error bound for a computed solution x^, such that x^-x1/x1errbnd, where x^ is a column of the computed solution returned in the array b and x is the corresponding column of the exact solution X. If rcond is less than machine precision, errbnd is returned as unity.
10:   ifail – IntegerInput/Output
On entry: ifail must be set to 0, -1​ or ​1. If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation 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).

Error Indicators and Warnings

If on entry ifail=0 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
Diagonal block value of the block diagonal matrix is zero. The factorization has been completed, but the solution could not be computed.
A solution has been computed, but rcond is less than machine precision so that the matrix A is numerically singular.
On entry, uplo not one of 'U' or 'u' or 'L' or 'l': uplo=value.
On entry, n=value.
Constraint: n0.
On entry, nrhs=value.
Constraint: nrhs0.
On entry, ldb=value and n=value.
Constraint: ldbmax1,n.
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
Dynamic memory allocation failed.
The integer allocatable memory required is n, and the real allocatable memory required is 2×n. Allocation failed before the solution could be computed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.


The computed solution for a single right-hand side, x^, satisfies an equation of the form
A+E x^=b,  
E1 = Oε A1  
and ε is the machine precision. An approximate error bound for the computed solution is given by
x^-x1 x1 κA E1 A1 ,  
where κA = A-11 A1 , the condition number of A with respect to the solution of the linear equations. f04bjf uses the approximation E1=εA1 to estimate errbnd. See Section 4.4 of Anderson et al. (1999) for further details.

Parallelism and Performance

f04bjf 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.

Further Comments

The packed storage scheme is illustrated by the following example when n=4 and uplo='U'. Two-dimensional storage of the symmetric matrix A:
a11 a12 a13 a14 a22 a23 a24 a33 a34 a44 aij = aji  
Packed storage of the upper triangle of A:
ap= a11, a12, a22, a13, a23, a33, a14, a24, a34, a44  
The total number of floating-point operations required to solve the equations AX=B is proportional to 13n3+2n2r. The condition number estimation typically requires between four and five solves and never more than eleven solves, following the factorization.
In practice the condition number estimator is very reliable, but it can underestimate the true condition number; see Section 15.3 of Higham (2002) for further details.
The complex analogues of f04bjf are f04cjf for complex Hermitian matrices, and f04djf for complex symmetric matrices.


This example solves the equations
where A is the symmetric indefinite matrix
A= -1.81 2.06 0.63 -1.15 2.06 1.15 1.87 4.20 0.63 1.87 -0.21 3.87 -1.15 4.20 3.87 2.07   and   B= 0.96 3.93 6.07 19.25 8.38 9.90 9.50 27.85 .  
An estimate of the condition number of A and an approximate error bound for the computed solutions are also printed.

Program Text

Program Text (f04bjfe.f90)

Program Data

Program Data (f04bjfe.d)

Program Results

Program Results (f04bjfe.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017