# cstein.f man page

cstein.f

## Synopsis

### Functions/Subroutines

subroutine **cstein** (**N**, D, E, M, W, IBLOCK, ISPLIT, Z, LDZ, WORK, IWORK, IFAIL, INFO)**CSTEIN**

## Function/Subroutine Documentation

### subroutine cstein (integer N, real, dimension( * ) D, real, dimension( * ) E, integer M, real, dimension( * ) W, integer, dimension( * ) IBLOCK, integer, dimension( * ) ISPLIT, complex, dimension( ldz, * ) Z, integer LDZ, real, dimension( * ) WORK, integer, dimension( * ) IWORK, integer, dimension( * ) IFAIL, integer INFO)

**CSTEIN**

**Purpose:**

CSTEIN 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 CUNMTR or CUPMTR 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 REAL array, dimension (N) The n diagonal elements of the tridiagonal matrix T.

*E*E is REAL 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 REAL 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 SSTEBZ 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 SSTEBZ 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 SSTEBZ is expected here. )

*Z*Z is COMPLEX 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 REAL 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:**December 2016

Definition at line 184 of file cstein.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page cstein(3) is an alias of cstein.f(3).

Tue Nov 14 2017 Version 3.8.0 LAPACK