!>
!> 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.
!>
!> 

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.
!> 

!>  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.
!> 

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