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zstein.f(3) LAPACK zstein.f(3)

zstein.f -


subroutine zstein (N, D, E, M, W, IBLOCK, ISPLIT, Z, LDZ, WORK, IWORK, IFAIL, INFO)
 
ZSTEIN

ZSTEIN
Purpose:
 ZSTEIN computes the eigenvectors of a real symmetric tridiagonal
 matrix T corresponding to specified eigenvalues, using inverse
 iteration.
The maximum number of iterations allowed for each eigenvector is specified by an internal parameter MAXITS (currently set to 5).
Although the eigenvectors are real, they are stored in a complex array, which may be passed to ZUNMTR or ZUPMTR for back transformation to the eigenvectors of a complex Hermitian matrix which was reduced to tridiagonal form.
Parameters:
N
          N is INTEGER
          The order of the matrix.  N >= 0.
D
          D is DOUBLE PRECISION array, dimension (N)
          The n diagonal elements of the tridiagonal matrix T.
E
          E is DOUBLE PRECISION array, dimension (N-1)
          The (n-1) subdiagonal elements of the tridiagonal matrix
          T, stored in elements 1 to N-1.
M
          M is INTEGER
          The number of eigenvectors to be found.  0 <= M <= N.
W
          W is DOUBLE PRECISION array, dimension (N)
          The first M elements of W contain the eigenvalues for
          which eigenvectors are to be computed.  The eigenvalues
          should be grouped by split-off block and ordered from
          smallest to largest within the block.  ( The output array
          W from DSTEBZ with ORDER = 'B' is expected here. )
IBLOCK
          IBLOCK is INTEGER array, dimension (N)
          The submatrix indices associated with the corresponding
          eigenvalues in W; IBLOCK(i)=1 if eigenvalue W(i) belongs to
          the first submatrix from the top, =2 if W(i) belongs to
          the second submatrix, etc.  ( The output array IBLOCK
          from DSTEBZ is expected here. )
ISPLIT
          ISPLIT is INTEGER array, dimension (N)
          The splitting points, at which T breaks up into submatrices.
          The first submatrix consists of rows/columns 1 to
          ISPLIT( 1 ), the second of rows/columns ISPLIT( 1 )+1
          through ISPLIT( 2 ), etc.
          ( The output array ISPLIT from DSTEBZ is expected here. )
Z
          Z is COMPLEX*16 array, dimension (LDZ, M)
          The computed eigenvectors.  The eigenvector associated
          with the eigenvalue W(i) is stored in the i-th column of
          Z.  Any vector which fails to converge is set to its current
          iterate after MAXITS iterations.
          The imaginary parts of the eigenvectors are set to zero.
LDZ
          LDZ is INTEGER
          The leading dimension of the array Z.  LDZ >= max(1,N).
WORK
          WORK is DOUBLE PRECISION array, dimension (5*N)
IWORK
          IWORK is INTEGER array, dimension (N)
IFAIL
          IFAIL is INTEGER array, dimension (M)
          On normal exit, all elements of IFAIL are zero.
          If one or more eigenvectors fail to converge after
          MAXITS iterations, then their indices are stored in
          array IFAIL.
INFO
          INFO is INTEGER
          = 0: successful exit
          < 0: if INFO = -i, the i-th argument had an illegal value
          > 0: if INFO = i, then i eigenvectors failed to converge
               in MAXITS iterations.  Their indices are stored in
               array IFAIL.
Internal Parameters:
  MAXITS  INTEGER, default = 5
          The maximum number of iterations performed.
EXTRA INTEGER, default = 2 The number of iterations performed after norm growth criterion is satisfied, should be at least 1.
Author:
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
November 2011
Definition at line 182 of file zstein.f.

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