cstegr.f.html
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SUBROUTINE <a name="CSTEGR.1"></a><a href="cstegr.f.html#CSTEGR.1">CSTEGR</a>( JOBZ, RANGE, N, D, E, VL, VU, IL, IU,
$ ABSTOL, M, W, Z, LDZ, ISUPPZ, WORK, LWORK, IWORK,
$ LIWORK, INFO )
IMPLICIT NONE
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> -- LAPACK computational routine (version 3.1) --
</span><span class="comment">*</span><span class="comment"> Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
</span><span class="comment">*</span><span class="comment"> November 2006
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> .. Scalar Arguments ..
</span> CHARACTER JOBZ, RANGE
INTEGER IL, INFO, IU, LDZ, LIWORK, LWORK, M, N
REAL ABSTOL, VL, VU
<span class="comment">*</span><span class="comment"> ..
</span><span class="comment">*</span><span class="comment"> .. Array Arguments ..
</span> INTEGER ISUPPZ( * ), IWORK( * )
REAL D( * ), E( * ), W( * ), WORK( * )
COMPLEX Z( LDZ, * )
<span class="comment">*</span><span class="comment"> ..
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> Purpose
</span><span class="comment">*</span><span class="comment"> =======
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> <a name="CSTEGR.26"></a><a href="cstegr.f.html#CSTEGR.1">CSTEGR</a> computes selected eigenvalues and, optionally, eigenvectors
</span><span class="comment">*</span><span class="comment"> of a real symmetric tridiagonal matrix T. Any such unreduced matrix has
</span><span class="comment">*</span><span class="comment"> a well defined set of pairwise different real eigenvalues, the corresponding
</span><span class="comment">*</span><span class="comment"> real eigenvectors are pairwise orthogonal.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> The spectrum may be computed either completely or partially by specifying
</span><span class="comment">*</span><span class="comment"> either an interval (VL,VU] or a range of indices IL:IU for the desired
</span><span class="comment">*</span><span class="comment"> eigenvalues.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> <a name="CSTEGR.35"></a><a href="cstegr.f.html#CSTEGR.1">CSTEGR</a> is a compatability wrapper around the improved <a name="CSTEMR.35"></a><a href="cstemr.f.html#CSTEMR.1">CSTEMR</a> routine.
</span><span class="comment">*</span><span class="comment"> See <a name="SSTEMR.36"></a><a href="sstemr.f.html#SSTEMR.1">SSTEMR</a> for further details.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> One important change is that the ABSTOL parameter no longer provides any
</span><span class="comment">*</span><span class="comment"> benefit and hence is no longer used.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> Note : <a name="CSTEGR.41"></a><a href="cstegr.f.html#CSTEGR.1">CSTEGR</a> and <a name="CSTEMR.41"></a><a href="cstemr.f.html#CSTEMR.1">CSTEMR</a> work only on machines which follow
</span><span class="comment">*</span><span class="comment"> IEEE-754 floating-point standard in their handling of infinities and
</span><span class="comment">*</span><span class="comment"> NaNs. Normal execution may create these exceptiona values and hence
</span><span class="comment">*</span><span class="comment"> may abort due to a floating point exception in environments which
</span><span class="comment">*</span><span class="comment"> do not conform to the IEEE-754 standard.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> Arguments
</span><span class="comment">*</span><span class="comment"> =========
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> JOBZ (input) CHARACTER*1
</span><span class="comment">*</span><span class="comment"> = 'N': Compute eigenvalues only;
</span><span class="comment">*</span><span class="comment"> = 'V': Compute eigenvalues and eigenvectors.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> RANGE (input) CHARACTER*1
</span><span class="comment">*</span><span class="comment"> = 'A': all eigenvalues will be found.
</span><span class="comment">*</span><span class="comment"> = 'V': all eigenvalues in the half-open interval (VL,VU]
</span><span class="comment">*</span><span class="comment"> will be found.
</span><span class="comment">*</span><span class="comment"> = 'I': the IL-th through IU-th eigenvalues will be found.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> N (input) INTEGER
</span><span class="comment">*</span><span class="comment"> The order of the matrix. N >= 0.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> D (input/output) REAL array, dimension (N)
</span><span class="comment">*</span><span class="comment"> On entry, the N diagonal elements of the tridiagonal matrix
</span><span class="comment">*</span><span class="comment"> T. On exit, D is overwritten.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> E (input/output) REAL array, dimension (N)
</span><span class="comment">*</span><span class="comment"> On entry, the (N-1) subdiagonal elements of the tridiagonal
</span><span class="comment">*</span><span class="comment"> matrix T in elements 1 to N-1 of E. E(N) need not be set on
</span><span class="comment">*</span><span class="comment"> input, but is used internally as workspace.
</span><span class="comment">*</span><span class="comment"> On exit, E is overwritten.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> VL (input) REAL
</span><span class="comment">*</span><span class="comment"> VU (input) REAL
</span><span class="comment">*</span><span class="comment"> If RANGE='V', the lower and upper bounds of the interval to
</span><span class="comment">*</span><span class="comment"> be searched for eigenvalues. VL < VU.
</span><span class="comment">*</span><span class="comment"> Not referenced if RANGE = 'A' or 'I'.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment"> IL (input) INTEGER
</span><span class="comment">*</span><span class="comment"> IU (input) INTEGER
</span><span class="comment">*</span><span class="comment"> If RANGE='I', the indices (in ascending order) of the
</span><span class="comment">*</span><span class="comment"> smallest and largest eigenvalues to be returned.
</span><span class="comment">*</span><span class="comment"> 1 <= IL <= IU <= N, if N > 0.
</span><span class="comment">*</span><span class="comment"> Not referenced if RANGE = 'A' or 'V'.
</span><span class="comment">*</span><span class="comment">
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