NAG Library Routine Document

f04dhf  (complex_symm_solve)

 Contents

    1  Purpose
    7  Accuracy

1
Purpose

f04dhf computes the solution to a complex system of linear equations AX=B, where A is an n by n complex symmetric matrix 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.

2
Specification

Fortran Interface
Subroutine f04dhf ( uplo, n, nrhs, a, lda, ipiv, b, ldb, rcond, errbnd, ifail)
Integer, Intent (In):: n, nrhs, lda, ldb
Integer, Intent (Inout):: ifail
Integer, Intent (Out):: ipiv(n)
Real (Kind=nag_wp), Intent (Out):: rcond, errbnd
Complex (Kind=nag_wp), Intent (Inout):: a(lda,*), b(ldb,*)
Character (1), Intent (In):: uplo
C Header Interface
#include nagmk26.h
void  f04dhf_ ( const char *uplo, const Integer *n, const Integer *nrhs, Complex a[], const Integer *lda, Integer ipiv[], Complex b[], const Integer *ldb, double *rcond, double *errbnd, Integer *ifail, const Charlen length_uplo)

3
Description

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.

4
References

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 http://www.netlib.org/lapack/lug
Higham N J (2002) Accuracy and Stability of Numerical Algorithms (2nd Edition) SIAM, Philadelphia

5
Arguments

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:     alda* – Complex (Kind=nag_wp) arrayInput/Output
Note: the second dimension of the array a must be at least max1,n.
On entry: the n by n complex symmetric matrix A.
If uplo='U', the leading n by n upper triangular part of the array a contains the upper triangular part of the matrix A, and the strictly lower triangular part of a is not referenced.
If uplo='L', the leading n by n lower triangular part of the array a contains the lower triangular part of the matrix A, and the strictly upper triangular part of a is not referenced.
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 f07nrf (zsytrf).
5:     lda – IntegerInput
On entry: the first dimension of the array a as declared in the (sub)program from which f04dhf is called.
Constraint: ldamax1,n.
6:     ipivn – Integer arrayOutput
On exit: if ifail0, details of the interchanges and the block structure of D, as determined by f07nrf (zsytrf).
  • 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.
7:     bldb* – Complex (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.
8:     ldb – IntegerInput
On entry: the first dimension of the array b as declared in the (sub)program from which f04dhf is called.
Constraint: ldbmax1,n.
9:     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.
10:   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.
11:   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).

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:
ifail>0andifailn
Diagonal block value of the block diagonal matrix is zero. The factorization has been completed, but the solution could not be computed.
ifail=n+1
A solution has been computed, but rcond is less than machine precision so that the matrix A is numerically singular.
ifail=-1
On entry, uplo not one of 'U' or 'u' or 'L' or 'l': uplo=value.
ifail=-2
On entry, n=value.
Constraint: n0.
ifail=-3
On entry, nrhs=value.
Constraint: nrhs0.
ifail=-5
On entry, lda=value and n=value.
Constraint: ldamax1,n.
ifail=-8
On entry, ldb=value and n=value.
Constraint: ldbmax1,n.
ifail=-99
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.
ifail=-399
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.
ifail=-999
Dynamic memory allocation failed.
The real allocatable memory required is n, and the complex allocatable memory required is max2×n,lwork, where lwork is the optimum workspace required by f07nnf (zsysv). If this failure occurs it may be possible to solve the equations by calling the packed storage version of f04dhf, f04djf, or by calling f07nnf (zsysv) directly with less than the optimum workspace (see Chapter F07).
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

7
Accuracy

The computed solution for a single right-hand side, x^, satisfies an equation of the form
A+E x^ = b ,  
where
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. f04dhf uses the approximation E1=εA1 to estimate errbnd. See Section 4.4 of Anderson et al. (1999) for further details.

8
Parallelism and Performance

f04dhf 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

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.
Routine f04chf is for complex Hermitian matrices, and the real analogue of f04dhf is f04bhf.

10
Example

This example solves the equations
AX=B,  
where A is the symmetric indefinite matrix
A= -0.56+0.12i -1.54-2.86i 5.32-1.59i 3.80+0.92i -1.54-2.86i -2.83-0.03i -3.52+0.58i -7.86-2.96i 5.32-1.59i -3.52+0.58i 8.86+1.81i 5.14-0.64i 3.80+0.92i -7.86-2.96i 5.14-0.64i -0.39-0.71i  
and
B= -6.43+19.24i -4.59-35.53i -0.49-01.47i 6.95+20.49i -48.18+66.00i -12.08-27.02i -55.64+41.22i -19.09-35.97i .  
An estimate of the condition number of A and an approximate error bound for the computed solutions are also printed.

10.1
Program Text

Program Text (f04dhfe.f90)

10.2
Program Data

Program Data (f04dhfe.d)

10.3
Program Results

Program Results (f04dhfe.r)

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