LAPACK  3.4.2
LAPACK: Linear Algebra PACKage
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ssteqr.f File Reference

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Functions/Subroutines

subroutine ssteqr (COMPZ, N, D, E, Z, LDZ, WORK, INFO)
 SSTEQR

Function/Subroutine Documentation

subroutine ssteqr ( character  COMPZ,
integer  N,
real, dimension( * )  D,
real, dimension( * )  E,
real, dimension( ldz, * )  Z,
integer  LDZ,
real, dimension( * )  WORK,
integer  INFO 
)

SSTEQR

Download SSTEQR + dependencies [TGZ] [ZIP] [TXT]
Purpose: 
 SSTEQR computes all eigenvalues and, optionally, eigenvectors of a
 symmetric tridiagonal matrix using the implicit QL or QR method.
 The eigenvectors of a full or band symmetric matrix can also be found
 if SSYTRD or SSPTRD or SSBTRD has been used to reduce this matrix to
 tridiagonal form.
Parameters:
[in]COMPZ
          COMPZ is CHARACTER*1
          = 'N':  Compute eigenvalues only.
          = 'V':  Compute eigenvalues and eigenvectors of the original
                  symmetric matrix.  On entry, Z must contain the
                  orthogonal matrix used to reduce the original matrix
                  to tridiagonal form.
          = 'I':  Compute eigenvalues and eigenvectors of the
                  tridiagonal matrix.  Z is initialized to the identity
                  matrix.
[in]N
          N is INTEGER
          The order of the matrix.  N >= 0.
[in,out]D
          D is REAL array, dimension (N)
          On entry, the diagonal elements of the tridiagonal matrix.
          On exit, if INFO = 0, the eigenvalues in ascending order.
[in,out]E
          E is REAL array, dimension (N-1)
          On entry, the (n-1) subdiagonal elements of the tridiagonal
          matrix.
          On exit, E has been destroyed.
[in,out]Z
          Z is REAL array, dimension (LDZ, N)
          On entry, if  COMPZ = 'V', then Z contains the orthogonal
          matrix used in the reduction to tridiagonal form.
          On exit, if INFO = 0, then if  COMPZ = 'V', Z contains the
          orthonormal eigenvectors of the original symmetric matrix,
          and if COMPZ = 'I', Z contains the orthonormal eigenvectors
          of the symmetric tridiagonal matrix.
          If COMPZ = 'N', then Z is not referenced.
[in]LDZ
          LDZ is INTEGER
          The leading dimension of the array Z.  LDZ >= 1, and if
          eigenvectors are desired, then  LDZ >= max(1,N).
[out]WORK
          WORK is REAL array, dimension (max(1,2*N-2))
          If COMPZ = 'N', then WORK is not referenced.
[out]INFO
          INFO is INTEGER
          = 0:  successful exit
          < 0:  if INFO = -i, the i-th argument had an illegal value
          > 0:  the algorithm has failed to find all the eigenvalues in
                a total of 30*N iterations; if INFO = i, then i
                elements of E have not converged to zero; on exit, D
                and E contain the elements of a symmetric tridiagonal
                matrix which is orthogonally similar to the original
                matrix.
Author:
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
November 2011

Definition at line 132 of file ssteqr.f.

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