Purpose ======= LA_STEVR computes selected eigenvalues and, optionally, the corresponding eigenvectors of a real symmetric tridiagonal matrix A. Eigenvalues and eigenvectors can be selected by specifying either a range of values or a range of indices for the desired eigenvalues. LA_STEVR uses a relatively robust representation (RRR) algorithm. It is usually the fastest algorithm of all and uses the least workspace. ========= SUBROUTINE LA_STEVR ( D, E, W, Z=z, VL=vl, VU=vu, & IL=il, IU=iu, M=m, ISUPPZ=isuppz, & ABSTOL=abstol, INFO=info ) REAL(), INTENT(INOUT) :: D(:), E(:) REAL(), INTENT(OUT) :: W(:) REAL(), INTENT(OUT), OPTIONAL :: Z(:,:) INTEGER, INTENT(OUT), OPTIONAL :: ISUPPZ(:) REAL(), INTENT(IN), OPTIONAL :: VL, VU INTEGER, INTENT(IN), OPTIONAL :: IL, IU INTEGER, INTENT(OUT), OPTIONAL :: M REAL(), INTENT(IN), OPTIONAL :: ABSTOL INTEGER, INTENT(OUT), OPTIONAL :: INFO where ::= KIND(1.0) | KIND(1.0D0) Arguments ========= D (input/output) REAL array, shape (:) with size(D) = n, where n is the order of A. On entry, the diagonal elements of the matrix A. On exit, the original contents of D possibly multiplied by a constant factor to avoid over/underflow in computing the eigenvalues. E (input/output) REAL array, shape (:) with size(E) = n. On entry, the n-1 subdiagonal elements of A in E(1) to E(n-1) . E(n) need not be set. On exit, the original contents of E possibly multiplied by a constant factor to avoid over/underflow in computing the eigenvalues. W (output) REAL array with size(W) = n. The first M elements contain the selected eigenvalues in ascending order. Z Optional (output) REAL or COMPLEX array, shape (:,:) with size(Z,1) = n and size(Z,2) = M. The first M columns of Z contain the orthonormal eigenvectors of A corresponding to the selected eigenvalues, with the i-th column of Z containing the eigenvector associated with the eigenvalue in W(i). Note: The user must ensure that at least M columns are supplied in the array Z. When the exact value of M is not known in advance, an upper bound must be used. In all cases M <= n. VL,VU Optional (input) REAL. The lower and upper bounds of the interval to be searched for eigenvalues. VL < VU. Default values: VL = -HUGE() and VU = HUGE(), where ::= KIND(1.0) | KIND(1.0D0). Note: Neither VL nor VU may be present if IL and/or IU is present. IL,IU Optional (input) INTEGER. The indices of the smallest and largest eigenvalues to be returned. The IL-th through IU-th eigenvalues will be found. 1 <= IL <= IU <= n. Default values: IL = 1 and IU = n. Note: Neither IL nor IU may be present if VL and/or VU is present. Note: All eigenvalues are calculated if none of the arguments VL, VU, IL and IU are present. M Optional (output) INTEGER. The total number of eigenvalues found. 0 <= M <= n. Note: If IL and IU are present then M = IU - IL + 1. ISUPPZ Optional (output) INTEGER array, shape (:) with size(ISUPPZ) = 2*max(1,M). The support of the eigenvectors in A, i.e., the indices indicating the nonzero elements. The i-th eigenvector is nonzero only in elements ISUPPZ(2*i-1) through ISUPPZ(2*i). ABSTOL Optional (input) REAL. The absolute error tolerance for the eigenvalues. An approximate eigenvalue is accepted as converged when it is determined to lie in an interval [a, b] of width less than or equal to ABSTOL + EPSILON(1.0_) * max(| a |, | b |), where is the working precision. If ABSTOL <= 0, then EPSILON(1.0_)*||A||1 will be used in its place. Eigenvalues will be computed most accurately if ABSTOL is set to LA_LAMCH( 1.0_, 'Safe minimum'), not zero. Default value: 0.0_. INFO Optional (output) INTEGER = 0: successful exit. < 0: if INFO = -i, the i-th argument had an illegal value. > 0: an internal error occurred. If INFO is not present and an error occurs, then the program is terminated with an error message.