NAG C Library Function Document

nag_matop_real_gen_matrix_fun_std (f01ekc)

1
Purpose

nag_matop_real_gen_matrix_fun_std (f01ekc) computes the matrix exponential, sine, cosine, sinh or cosh, of a real n by n matrix A using the Schur–Parlett algorithm.

2
Specification

#include <nag.h>
#include <nagf01.h>
void  nag_matop_real_gen_matrix_fun_std (Nag_OrderType order, Nag_MatFunType fun, Integer n, double a[], Integer pda, double *imnorm, NagError *fail)

3
Description

fA, where f is either the exponential, sine, cosine, sinh or cosh, is computed using the Schur–Parlett algorithm described in Higham (2008) and Davies and Higham (2003).

4
References

Davies P I and Higham N J (2003) A Schur–Parlett algorithm for computing matrix functions. SIAM J. Matrix Anal. Appl. 25(2) 464–485
Higham N J (2008) Functions of Matrices: Theory and Computation SIAM, Philadelphia, PA, USA

5
Arguments

1:     order Nag_OrderTypeInput
On entry: the order argument specifies the two-dimensional storage scheme being used, i.e., row-major ordering or column-major ordering. C language defined storage is specified by order=Nag_RowMajor. See Section 3.3.1.3 in How to Use the NAG Library and its Documentation for a more detailed explanation of the use of this argument.
Constraint: order=Nag_RowMajor or Nag_ColMajor.
2:     fun Nag_MatFunTypeInput
On entry: indicates which matrix function will be computed.
fun=Nag_Exp
The matrix exponential, eA, will be computed.
fun=Nag_Sin
The matrix sine, sinA, will be computed.
fun=Nag_Cos
The matrix cosine, cosA, will be computed.
fun=Nag_Sinh
The hyperbolic matrix sine, sinhA, will be computed.
fun=Nag_Cosh
The hyperbolic matrix cosine, coshA, will be computed.
Constraint: fun=Nag_Exp, Nag_Sin, Nag_Cos, Nag_Sinh or Nag_Cosh.
3:     n IntegerInput
On entry: n, the order of the matrix A.
Constraint: n0.
4:     a[dim] doubleInput/Output
Note: the dimension, dim, of the array a must be at least pda×n.
The i,jth element of the matrix A is stored in
  • a[j-1×pda+i-1] when order=Nag_ColMajor;
  • a[i-1×pda+j-1] when order=Nag_RowMajor.
On entry: the n by n matrix A.
On exit: the n by n matrix, fA.
5:     pda IntegerInput
On entry: the stride separating row or column elements (depending on the value of order) in the array a.
Constraint: pdan.
6:     imnorm double *Output
On exit: if A has complex eigenvalues, nag_matop_real_gen_matrix_fun_std (f01ekc) will use complex arithmetic to compute the matrix function. The imaginary part is discarded at the end of the computation, because it will theoretically vanish. imnorm contains the 1-norm of the imaginary part, which should be used to check that the routine has given a reliable answer.
If A has real eigenvalues, nag_matop_real_gen_matrix_fun_std (f01ekc) uses real arithmetic and imnorm=0.
7:     fail NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

6
Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
NE_BAD_PARAM
On entry, argument value had an illegal value.
NE_CONVERGENCE
A Taylor series failed to converge.
NE_INT
On entry, n=value.
Constraint: n0.
NE_INT_2
On entry, pda=value and n=value.
Constraint: pdan.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
An unexpected internal error occurred when evaluating the function at a point. Please contact NAG.
An unexpected internal error occurred when ordering the eigenvalues of A. Please contact NAG.
The function was unable to compute the Schur decomposition of A.
Note:  this failure should not occur and suggests that the function has been called incorrectly.
There was an error whilst reordering the Schur form of A.
Note:  this failure should not occur and suggests that the function has been called incorrectly.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
NE_SINGULAR
The linear equations to be solved are nearly singular and the Padé approximant used to compute the exponential may have no correct figures.
Note:  this failure should not occur and suggests that the function has been called incorrectly.

7
Accuracy

For a normal matrix A (for which ATA=AAT), the Schur decomposition is diagonal and the algorithm reduces to evaluating f at the eigenvalues of A and then constructing fA using the Schur vectors. This should give a very accurate result. In general, however, no error bounds are available for the algorithm.
For further discussion of the Schur–Parlett algorithm see Section 9.4 of Higham (2008).

8
Parallelism and Performance

nag_matop_real_gen_matrix_fun_std (f01ekc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_matop_real_gen_matrix_fun_std (f01ekc) 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 function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9
Further Comments

The Integer allocatable memory required is n. If A has real eigenvalues then up to 9n2 of double allocatable memory may be required. If A has complex eigenvalues then up to 9n2 of Complex allocatable memory may be required.
The cost of the Schur–Parlett algorithm depends on the spectrum of A, but is roughly between 28n3 and n4/3 floating-point operations; see Algorithm 9.6 of Higham (2008).
If the matrix exponential is required then it is recommended that nag_real_gen_matrix_exp (f01ecc) be used. nag_real_gen_matrix_exp (f01ecc) uses an algorithm which is, in general, more accurate than the Schur–Parlett algorithm used by nag_matop_real_gen_matrix_fun_std (f01ekc).
If estimates of the condition number of the matrix function are required then nag_matop_real_gen_matrix_cond_std (f01jac) should be used.
nag_matop_complex_gen_matrix_fun_std (f01fkc) can be used to find the matrix exponential, sin, cos, sinh or cosh of a complex matrix.

10
Example

This example finds the matrix cosine of the matrix
A = 2 0 1 0 0 2 -2 1 0 2 3 1 1 4 0 0 .  

10.1
Program Text

Program Text (f01ekce.c)

10.2
Program Data

Program Data (f01ekce.d)

10.3
Program Results

Program Results (f01ekce.r)