LAPACK

NAME¶

stevd - stevd: eig, divide and conquer

SYNOPSIS¶

Functions¶

subroutine dstevd (jobz, n, d, e, z, ldz, work, lwork, iwork, liwork, info)
DSTEVD computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices subroutine sstevd (jobz, n, d, e, z, ldz, work, lwork, iwork, liwork, info)
SSTEVD computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices

Detailed Description¶

Function Documentation¶

subroutine dstevd (character jobz, integer n, double precision, dimension( * ) d, double precision, dimension( * ) e, double precision, dimension( ldz, * ) z, integer ldz, double precision, dimension( * ) work, integer lwork, integer, dimension( * ) iwork, integer liwork, integer info)¶

DSTEVD computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices

Purpose:



 DSTEVD computes all eigenvalues and, optionally, eigenvectors of a


 real symmetric tridiagonal matrix. If eigenvectors are desired, it


 uses a divide and conquer algorithm.

Parameters

JOBZ



          JOBZ is CHARACTER*1


          = 'N':  Compute eigenvalues only;


          = 'V':  Compute eigenvalues and eigenvectors.



          N is INTEGER


          The order of the matrix.  N >= 0.



          D is DOUBLE PRECISION array, dimension (N)


          On entry, the n diagonal elements of the tridiagonal matrix


          A.


          On exit, if INFO = 0, the eigenvalues in ascending order.



          E is DOUBLE PRECISION array, dimension (N-1)


          On entry, the (n-1) subdiagonal elements of the tridiagonal


          matrix A, stored in elements 1 to N-1 of E.


          On exit, the contents of E are destroyed.



          Z is DOUBLE PRECISION array, dimension (LDZ, N)


          If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal


          eigenvectors of the matrix A, with the i-th column of Z


          holding the eigenvector associated with D(i).


          If JOBZ = 'N', then Z is not referenced.

LDZ



          LDZ is INTEGER


          The leading dimension of the array Z.  LDZ >= 1, and if


          JOBZ = 'V', LDZ >= max(1,N).

WORK



          WORK is DOUBLE PRECISION array,


                                         dimension (LWORK)


          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

LWORK



          LWORK is INTEGER


          The dimension of the array WORK.


          If JOBZ  = 'N' or N <= 1 then LWORK must be at least 1.


          If JOBZ  = 'V' and N > 1 then LWORK must be at least


                         ( 1 + 4*N + N**2 ).


          If LWORK = -1, then a workspace query is assumed; the routine


          only calculates the optimal sizes of the WORK and IWORK


          arrays, returns these values as the first entries of the WORK


          and IWORK arrays, and no error message related to LWORK or


          LIWORK is issued by XERBLA.

IWORK



          IWORK is INTEGER array, dimension (MAX(1,LIWORK))


          On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.

LIWORK



          LIWORK is INTEGER


          The dimension of the array IWORK.


          If JOBZ  = 'N' or N <= 1 then LIWORK must be at least 1.


          If JOBZ  = 'V' and N > 1 then LIWORK must be at least 3+5*N.


          If LIWORK = -1, then a workspace query is assumed; the


          routine only calculates the optimal sizes of the WORK and


          IWORK arrays, returns these values as the first entries of


          the WORK and IWORK arrays, and no error message related to


          LWORK or LIWORK is issued by XERBLA.

INFO



          INFO is INTEGER


          = 0:  successful exit


          < 0:  if INFO = -i, the i-th argument had an illegal value


          > 0:  if INFO = i, the algorithm failed to converge; i


                off-diagonal elements of E did not converge to zero.

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

subroutine sstevd (character jobz, integer n, real, dimension( * ) d, real, dimension( * ) e, real, dimension( ldz, * ) z, integer ldz, real, dimension( * ) work, integer lwork, integer, dimension( * ) iwork, integer liwork, integer info)¶

SSTEVD computes the eigenvalues and, optionally, the left and/or right eigenvectors for OTHER matrices