Computes the eigenvectors corresponding to specified eigenvalues of a real symmetric tridiagonal matrix, using inverse iteration.
call sstein2(n, d, e, m, w, iblock, isplit, orfac, z, ldz, work, iwork, ifail, info)
call dstein2(n, d, e, m, w, iblock, isplit, orfac, z, ldz, work, iwork, ifail, info)
The ?stein2 routine is a modified LAPACK routine ?stein. It 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).
INTEGER. The order of the matrix T (n ≥ 0).
INTEGER. The number of eigenvectors to be found (0 ≤ m ≤ n).
REAL for single-precision flavors
DOUBLE PRECISION for double-precision flavors.
Arrays: d(*), DIMENSION (n). The n diagonal elements of the tridiagonal matrix T.
e(*), DIMENSION (n).
The (n-1) subdiagonal elements of the tridiagonal matrix T, in elements 1 to n-1. e(n) need not be set.
w(*), 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 ?stebz with ORDER = 'B' is expected here).
The dimension of w must be at least max(1, n).
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,
iblock(i) = 2, if eigenvalue w(i) belongs to the second submatrix, etc. (The output array iblock from ?stebz is expected here).
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 submatrix consists of rows/columns isplit(1)+1 through isplit( 2 ), etc. (The output array isplit from ?stebz is expected here).
REAL for single-precision flavors
DOUBLE PRECISION for double-precision flavors.
orfac specifies which eigenvectors should be orthogonalized. Eigenvectors that correspond to eigenvalues which are within orfac*||T|| of each other are to be orthogonalized.
INTEGER. The leading dimension of the output array z; ldz ≥ max(1, n).
REAL for single-precision flavors
DOUBLE PRECISION for double-precision flavors.
Workspace array, DIMENSION (5n).
INTEGER. Workspace array, DIMENSION (n).
REAL for sstein2
DOUBLE PRECISION for dstein2
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 that fails to converge is set to its current iterate after maxits iterations.
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 the array ifail.
INTEGER.
info = 0, the exit is successful.
info < 0: if info = -i, the i-th had an illegal value.
info > 0: if info = i, then i eigenvectors failed to converge in maxits iterations. Their indices are stored in the array ifail.