zhsein (l)  Linux Manuals
zhsein: uses inverse iteration to find specified right and/or left eigenvectors of a complex upper Hessenberg matrix H
NAME
ZHSEIN  uses inverse iteration to find specified right and/or left eigenvectors of a complex upper Hessenberg matrix HSYNOPSIS
 SUBROUTINE ZHSEIN(
 SIDE, EIGSRC, INITV, SELECT, N, H, LDH, W, VL, LDVL, VR, LDVR, MM, M, WORK, RWORK, IFAILL, IFAILR, INFO )
 CHARACTER EIGSRC, INITV, SIDE
 INTEGER INFO, LDH, LDVL, LDVR, M, MM, N
 LOGICAL SELECT( * )
 INTEGER IFAILL( * ), IFAILR( * )
 DOUBLE PRECISION RWORK( * )
 COMPLEX*16 H( LDH, * ), VL( LDVL, * ), VR( LDVR, * ), W( * ), WORK( * )
PURPOSE
ZHSEIN uses inverse iteration to find specified right and/or left eigenvectors of a complex upper Hessenberg matrix H. The right eigenvector x and the left eigenvector y of the matrix H corresponding to an eigenvalue w are defined by:where y**h denotes the conjugate transpose of the vector y.
ARGUMENTS
 SIDE (input) CHARACTER*1

= aqRaq: compute right eigenvectors only;
= aqLaq: compute left eigenvectors only;
= aqBaq: compute both right and left eigenvectors.  EIGSRC (input) CHARACTER*1

Specifies the source of eigenvalues supplied in W:
= aqQaq: the eigenvalues were found using ZHSEQR; thus, if H has zero subdiagonal elements, and so is blocktriangular, then the jth eigenvalue can be assumed to be an eigenvalue of the block containing the jth row/column. This property allows ZHSEIN to perform inverse iteration on just one diagonal block. = aqNaq: no assumptions are made on the correspondence between eigenvalues and diagonal blocks. In this case, ZHSEIN must always perform inverse iteration using the whole matrix H.  INITV (input) CHARACTER*1

= aqNaq: no initial vectors are supplied;
= aqUaq: usersupplied initial vectors are stored in the arrays VL and/or VR.  SELECT (input) LOGICAL array, dimension (N)
 Specifies the eigenvectors to be computed. To select the eigenvector corresponding to the eigenvalue W(j), SELECT(j) must be set to .TRUE..
 N (input) INTEGER
 The order of the matrix H. N >= 0.
 H (input) COMPLEX*16 array, dimension (LDH,N)
 The upper Hessenberg matrix H.
 LDH (input) INTEGER
 The leading dimension of the array H. LDH >= max(1,N).
 W (input/output) COMPLEX*16 array, dimension (N)
 On entry, the eigenvalues of H. On exit, the real parts of W may have been altered since close eigenvalues are perturbed slightly in searching for independent eigenvectors.
 VL (input/output) COMPLEX*16 array, dimension (LDVL,MM)
 On entry, if INITV = aqUaq and SIDE = aqLaq or aqBaq, VL must contain starting vectors for the inverse iteration for the left eigenvectors; the starting vector for each eigenvector must be in the same column in which the eigenvector will be stored. On exit, if SIDE = aqLaq or aqBaq, the left eigenvectors specified by SELECT will be stored consecutively in the columns of VL, in the same order as their eigenvalues. If SIDE = aqRaq, VL is not referenced.
 LDVL (input) INTEGER
 The leading dimension of the array VL. LDVL >= max(1,N) if SIDE = aqLaq or aqBaq; LDVL >= 1 otherwise.
 VR (input/output) COMPLEX*16 array, dimension (LDVR,MM)
 On entry, if INITV = aqUaq and SIDE = aqRaq or aqBaq, VR must contain starting vectors for the inverse iteration for the right eigenvectors; the starting vector for each eigenvector must be in the same column in which the eigenvector will be stored. On exit, if SIDE = aqRaq or aqBaq, the right eigenvectors specified by SELECT will be stored consecutively in the columns of VR, in the same order as their eigenvalues. If SIDE = aqLaq, VR is not referenced.
 LDVR (input) INTEGER
 The leading dimension of the array VR. LDVR >= max(1,N) if SIDE = aqRaq or aqBaq; LDVR >= 1 otherwise.
 MM (input) INTEGER
 The number of columns in the arrays VL and/or VR. MM >= M.
 M (output) INTEGER
 The number of columns in the arrays VL and/or VR required to store the eigenvectors (= the number of .TRUE. elements in SELECT).
 WORK (workspace) COMPLEX*16 array, dimension (N*N)
 RWORK (workspace) DOUBLE PRECISION array, dimension (N)
 IFAILL (output) INTEGER array, dimension (MM)
 If SIDE = aqLaq or aqBaq, IFAILL(i) = j > 0 if the left eigenvector in the ith column of VL (corresponding to the eigenvalue w(j)) failed to converge; IFAILL(i) = 0 if the eigenvector converged satisfactorily. If SIDE = aqRaq, IFAILL is not referenced.
 IFAILR (output) INTEGER array, dimension (MM)
 If SIDE = aqRaq or aqBaq, IFAILR(i) = j > 0 if the right eigenvector in the ith column of VR (corresponding to the eigenvalue w(j)) failed to converge; IFAILR(i) = 0 if the eigenvector converged satisfactorily. If SIDE = aqLaq, IFAILR is not referenced.
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
> 0: if INFO = i, i is the number of eigenvectors which failed to converge; see IFAILL and IFAILR for further details.
FURTHER DETAILS
Each eigenvector is normalized so that the element of largest magnitude has magnitude 1; here the magnitude of a complex number (x,y) is taken to be x+y.