shseqr - compute the eigenvalues of a real upper Hessenberg matrix H and, optionally, the matrices T and Z from the Schur decomposition H = Z T Z**T, where T is an upper quasi-triangular matrix (the Schur form), and Z is the orthogonal matrix of Schur vectors
SUBROUTINE SHSEQR( JOB, COMPZ, N, ILO, IHI, H, LDH, WR, WI, Z, LDZ, * WORK, LWORK, INFO) CHARACTER * 1 JOB, COMPZ INTEGER N, ILO, IHI, LDH, LDZ, LWORK, INFO REAL H(LDH,*), WR(*), WI(*), Z(LDZ,*), WORK(*)
SUBROUTINE SHSEQR_64( JOB, COMPZ, N, ILO, IHI, H, LDH, WR, WI, Z, * LDZ, WORK, LWORK, INFO) CHARACTER * 1 JOB, COMPZ INTEGER*8 N, ILO, IHI, LDH, LDZ, LWORK, INFO REAL H(LDH,*), WR(*), WI(*), Z(LDZ,*), WORK(*)
SUBROUTINE HSEQR( JOB, COMPZ, N, ILO, IHI, H, [LDH], WR, WI, Z, [LDZ], * [WORK], [LWORK], [INFO]) CHARACTER(LEN=1) :: JOB, COMPZ INTEGER :: N, ILO, IHI, LDH, LDZ, LWORK, INFO REAL, DIMENSION(:) :: WR, WI, WORK REAL, DIMENSION(:,:) :: H, Z
SUBROUTINE HSEQR_64( JOB, COMPZ, N, ILO, IHI, H, [LDH], WR, WI, Z, * [LDZ], [WORK], [LWORK], [INFO]) CHARACTER(LEN=1) :: JOB, COMPZ INTEGER(8) :: N, ILO, IHI, LDH, LDZ, LWORK, INFO REAL, DIMENSION(:) :: WR, WI, WORK REAL, DIMENSION(:,:) :: H, Z
#include <sunperf.h>
void shseqr(char job, char compz, int n, int ilo, int ihi, float *h, int ldh, float *wr, float *wi, float *z, int ldz, int *info);
void shseqr_64(char job, char compz, long n, long ilo, long ihi, float *h, long ldh, float *wr, float *wi, float *z, long ldz, long *info);
shseqr computes the eigenvalues of a real upper Hessenberg matrix H and, optionally, the matrices T and Z from the Schur decomposition H = Z T Z**T, where T is an upper quasi-triangular matrix (the Schur form), and Z is the orthogonal matrix of Schur vectors.
Optionally Z may be postmultiplied into an input orthogonal matrix Q, so that this routine can give the Schur factorization of a matrix A which has been reduced to the Hessenberg form H by the orthogonal matrix Q: A = Q*H*Q**T = (QZ)*T*(QZ)**T.
= 'E': compute eigenvalues only;
= 'S': compute eigenvalues and the Schur form T.
= 'N': no Schur vectors are computed;
= 'I': Z is initialized to the unit matrix and the matrix Z of Schur vectors of H is returned; = 'V': Z must contain an orthogonal matrix Q on entry, and the product Q*Z is returned.
H(i,i)
= H(i+1,i+1)
and H(i+1,i)*H(i,i+1)
< 0. If JOB = 'E',
the contents of H are unspecified on exit.
WI(i)
> 0 and
WI(i+1)
< 0. If JOB = 'S', the eigenvalues are stored in the
same order as on the diagonal of the Schur form returned in
H, with WR(i)
= H(i,i)
and, if H(i:i+1,i:i+1)
is a 2-by-2
diagonal block, WI(i)
= sqrt(H(i+1,i)*H(i,i+1))
and
WI(i+1)
= -WI(i).
If COMPZ = 'I': on entry, Z need not be set, and on exit, Z contains the orthogonal matrix Z of the Schur vectors of H. If COMPZ = 'V': on entry Z must contain an N-by-N matrix Q, which is assumed to be equal to the unit matrix except for the submatrix Z(ILO:IHI,ILO:IHI); on exit Z contains Q*Z. Normally Q is the orthogonal matrix generated by SORGHR after the call to SGEHRD which formed the Hessenberg matrix H.
max(1,N)
if COMPZ = 'I' or 'V'; LDZ > = 1 otherwise.
WORK(1)
returns the optimal LWORK.
If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA.
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = i, SHSEQR failed to compute all of the eigenvalues in a total of 30*(IHI-ILO+1) iterations; elements 1:ilo-1 and i+1:n of WR and WI contain those eigenvalues which have been successfully computed.