337 lines
11 KiB
Fortran
337 lines
11 KiB
Fortran
*> \brief \b SLATRD reduces the first nb rows and columns of a symmetric/Hermitian matrix A to real tridiagonal form by an orthogonal similarity transformation.
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*
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* =========== DOCUMENTATION ===========
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*
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* Online html documentation available at
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* http://www.netlib.org/lapack/explore-html/
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*
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*> \htmlonly
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*> Download SLATRD + dependencies
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/slatrd.f">
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*> [TGZ]</a>
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/slatrd.f">
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*> [ZIP]</a>
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*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/slatrd.f">
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*> [TXT]</a>
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*> \endhtmlonly
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*
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* Definition:
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* ===========
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*
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* SUBROUTINE SLATRD( UPLO, N, NB, A, LDA, E, TAU, W, LDW )
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*
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* .. Scalar Arguments ..
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* CHARACTER UPLO
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* INTEGER LDA, LDW, N, NB
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* ..
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* .. Array Arguments ..
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* REAL A( LDA, * ), E( * ), TAU( * ), W( LDW, * )
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* ..
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*
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*
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*> \par Purpose:
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* =============
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*>
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*> \verbatim
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*>
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*> SLATRD reduces NB rows and columns of a real symmetric matrix A to
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*> symmetric tridiagonal form by an orthogonal similarity
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*> transformation Q**T * A * Q, and returns the matrices V and W which are
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*> needed to apply the transformation to the unreduced part of A.
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*>
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*> If UPLO = 'U', SLATRD reduces the last NB rows and columns of a
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*> matrix, of which the upper triangle is supplied;
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*> if UPLO = 'L', SLATRD reduces the first NB rows and columns of a
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*> matrix, of which the lower triangle is supplied.
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*>
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*> This is an auxiliary routine called by SSYTRD.
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*> \endverbatim
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*
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* Arguments:
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* ==========
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*
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*> \param[in] UPLO
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*> \verbatim
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*> UPLO is CHARACTER*1
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*> Specifies whether the upper or lower triangular part of the
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*> symmetric matrix A is stored:
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*> = 'U': Upper triangular
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*> = 'L': Lower triangular
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*> \endverbatim
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*>
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*> \param[in] N
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*> \verbatim
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*> N is INTEGER
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*> The order of the matrix A.
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*> \endverbatim
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*>
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*> \param[in] NB
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*> \verbatim
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*> NB is INTEGER
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*> The number of rows and columns to be reduced.
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*> \endverbatim
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*>
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*> \param[in,out] A
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*> \verbatim
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*> A is REAL array, dimension (LDA,N)
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*> On entry, the symmetric matrix A. If UPLO = 'U', the leading
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*> n-by-n upper triangular part of A contains the upper
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*> triangular part of the matrix A, and the strictly lower
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*> triangular part of A is not referenced. If UPLO = 'L', the
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*> leading n-by-n lower triangular part of A contains the lower
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*> triangular part of the matrix A, and the strictly upper
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*> triangular part of A is not referenced.
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*> On exit:
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*> if UPLO = 'U', the last NB columns have been reduced to
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*> tridiagonal form, with the diagonal elements overwriting
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*> the diagonal elements of A; the elements above the diagonal
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*> with the array TAU, represent the orthogonal matrix Q as a
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*> product of elementary reflectors;
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*> if UPLO = 'L', the first NB columns have been reduced to
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*> tridiagonal form, with the diagonal elements overwriting
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*> the diagonal elements of A; the elements below the diagonal
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*> with the array TAU, represent the orthogonal matrix Q as a
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*> product of elementary reflectors.
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*> See Further Details.
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*> \endverbatim
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*>
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*> \param[in] LDA
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*> \verbatim
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*> LDA is INTEGER
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*> The leading dimension of the array A. LDA >= (1,N).
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*> \endverbatim
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*>
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*> \param[out] E
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*> \verbatim
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*> E is REAL array, dimension (N-1)
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*> If UPLO = 'U', E(n-nb:n-1) contains the superdiagonal
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*> elements of the last NB columns of the reduced matrix;
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*> if UPLO = 'L', E(1:nb) contains the subdiagonal elements of
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*> the first NB columns of the reduced matrix.
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*> \endverbatim
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*>
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*> \param[out] TAU
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*> \verbatim
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*> TAU is REAL array, dimension (N-1)
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*> The scalar factors of the elementary reflectors, stored in
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*> TAU(n-nb:n-1) if UPLO = 'U', and in TAU(1:nb) if UPLO = 'L'.
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*> See Further Details.
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*> \endverbatim
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*>
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*> \param[out] W
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*> \verbatim
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*> W is REAL array, dimension (LDW,NB)
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*> The n-by-nb matrix W required to update the unreduced part
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*> of A.
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*> \endverbatim
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*>
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*> \param[in] LDW
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*> \verbatim
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*> LDW is INTEGER
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*> The leading dimension of the array W. LDW >= max(1,N).
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*> \endverbatim
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*
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* Authors:
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* ========
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*
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*> \author Univ. of Tennessee
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*> \author Univ. of California Berkeley
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*> \author Univ. of Colorado Denver
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*> \author NAG Ltd.
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*
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*> \date September 2012
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*
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*> \ingroup doubleOTHERauxiliary
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*
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*> \par Further Details:
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* =====================
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*>
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*> \verbatim
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*>
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*> If UPLO = 'U', the matrix Q is represented as a product of elementary
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*> reflectors
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*>
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*> Q = H(n) H(n-1) . . . H(n-nb+1).
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*>
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*> Each H(i) has the form
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*>
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*> H(i) = I - tau * v * v**T
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*>
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*> where tau is a real scalar, and v is a real vector with
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*> v(i:n) = 0 and v(i-1) = 1; v(1:i-1) is stored on exit in A(1:i-1,i),
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*> and tau in TAU(i-1).
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*>
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*> If UPLO = 'L', the matrix Q is represented as a product of elementary
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*> reflectors
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*>
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*> Q = H(1) H(2) . . . H(nb).
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*>
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*> Each H(i) has the form
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*>
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*> H(i) = I - tau * v * v**T
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*>
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*> where tau is a real scalar, and v is a real vector with
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*> v(1:i) = 0 and v(i+1) = 1; v(i+1:n) is stored on exit in A(i+1:n,i),
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*> and tau in TAU(i).
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*>
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*> The elements of the vectors v together form the n-by-nb matrix V
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*> which is needed, with W, to apply the transformation to the unreduced
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*> part of the matrix, using a symmetric rank-2k update of the form:
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*> A := A - V*W**T - W*V**T.
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*>
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*> The contents of A on exit are illustrated by the following examples
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*> with n = 5 and nb = 2:
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*>
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*> if UPLO = 'U': if UPLO = 'L':
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*>
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*> ( a a a v4 v5 ) ( d )
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*> ( a a v4 v5 ) ( 1 d )
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*> ( a 1 v5 ) ( v1 1 a )
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*> ( d 1 ) ( v1 v2 a a )
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*> ( d ) ( v1 v2 a a a )
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*>
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*> where d denotes a diagonal element of the reduced matrix, a denotes
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*> an element of the original matrix that is unchanged, and vi denotes
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*> an element of the vector defining H(i).
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*> \endverbatim
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*>
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* =====================================================================
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SUBROUTINE SLATRD( UPLO, N, NB, A, LDA, E, TAU, W, LDW )
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*
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* -- LAPACK auxiliary routine (version 3.4.2) --
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* -- LAPACK is a software package provided by Univ. of Tennessee, --
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* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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* September 2012
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*
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* .. Scalar Arguments ..
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CHARACTER UPLO
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INTEGER LDA, LDW, N, NB
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* ..
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* .. Array Arguments ..
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REAL A( LDA, * ), E( * ), TAU( * ), W( LDW, * )
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* ..
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*
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* =====================================================================
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*
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* .. Parameters ..
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REAL ZERO, ONE, HALF
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PARAMETER ( ZERO = 0.0E+0, ONE = 1.0E+0, HALF = 0.5E+0 )
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* ..
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* .. Local Scalars ..
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INTEGER I, IW
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REAL ALPHA
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* ..
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* .. External Subroutines ..
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EXTERNAL SAXPY, SGEMV, SLARFG, SSCAL, SSYMV
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* ..
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* .. External Functions ..
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LOGICAL LSAME
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REAL SDOT
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EXTERNAL LSAME, SDOT
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC MIN
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* ..
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* .. Executable Statements ..
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*
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* Quick return if possible
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*
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IF( N.LE.0 )
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$ RETURN
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*
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IF( LSAME( UPLO, 'U' ) ) THEN
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*
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* Reduce last NB columns of upper triangle
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*
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DO 10 I = N, N - NB + 1, -1
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IW = I - N + NB
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IF( I.LT.N ) THEN
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*
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* Update A(1:i,i)
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*
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CALL SGEMV( 'No transpose', I, N-I, -ONE, A( 1, I+1 ),
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$ LDA, W( I, IW+1 ), LDW, ONE, A( 1, I ), 1 )
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CALL SGEMV( 'No transpose', I, N-I, -ONE, W( 1, IW+1 ),
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$ LDW, A( I, I+1 ), LDA, ONE, A( 1, I ), 1 )
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END IF
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IF( I.GT.1 ) THEN
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*
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* Generate elementary reflector H(i) to annihilate
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* A(1:i-2,i)
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*
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CALL SLARFG( I-1, A( I-1, I ), A( 1, I ), 1, TAU( I-1 ) )
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E( I-1 ) = A( I-1, I )
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A( I-1, I ) = ONE
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*
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* Compute W(1:i-1,i)
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*
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CALL SSYMV( 'Upper', I-1, ONE, A, LDA, A( 1, I ), 1,
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$ ZERO, W( 1, IW ), 1 )
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IF( I.LT.N ) THEN
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CALL SGEMV( 'Transpose', I-1, N-I, ONE, W( 1, IW+1 ),
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$ LDW, A( 1, I ), 1, ZERO, W( I+1, IW ), 1 )
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CALL SGEMV( 'No transpose', I-1, N-I, -ONE,
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$ A( 1, I+1 ), LDA, W( I+1, IW ), 1, ONE,
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$ W( 1, IW ), 1 )
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CALL SGEMV( 'Transpose', I-1, N-I, ONE, A( 1, I+1 ),
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$ LDA, A( 1, I ), 1, ZERO, W( I+1, IW ), 1 )
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CALL SGEMV( 'No transpose', I-1, N-I, -ONE,
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$ W( 1, IW+1 ), LDW, W( I+1, IW ), 1, ONE,
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$ W( 1, IW ), 1 )
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END IF
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CALL SSCAL( I-1, TAU( I-1 ), W( 1, IW ), 1 )
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ALPHA = -HALF*TAU( I-1 )*SDOT( I-1, W( 1, IW ), 1,
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$ A( 1, I ), 1 )
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CALL SAXPY( I-1, ALPHA, A( 1, I ), 1, W( 1, IW ), 1 )
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END IF
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*
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10 CONTINUE
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ELSE
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*
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* Reduce first NB columns of lower triangle
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*
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DO 20 I = 1, NB
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*
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* Update A(i:n,i)
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*
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CALL SGEMV( 'No transpose', N-I+1, I-1, -ONE, A( I, 1 ),
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$ LDA, W( I, 1 ), LDW, ONE, A( I, I ), 1 )
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CALL SGEMV( 'No transpose', N-I+1, I-1, -ONE, W( I, 1 ),
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$ LDW, A( I, 1 ), LDA, ONE, A( I, I ), 1 )
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IF( I.LT.N ) THEN
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*
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* Generate elementary reflector H(i) to annihilate
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* A(i+2:n,i)
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*
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CALL SLARFG( N-I, A( I+1, I ), A( MIN( I+2, N ), I ), 1,
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$ TAU( I ) )
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E( I ) = A( I+1, I )
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A( I+1, I ) = ONE
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*
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* Compute W(i+1:n,i)
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*
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CALL SSYMV( 'Lower', N-I, ONE, A( I+1, I+1 ), LDA,
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$ A( I+1, I ), 1, ZERO, W( I+1, I ), 1 )
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CALL SGEMV( 'Transpose', N-I, I-1, ONE, W( I+1, 1 ), LDW,
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$ A( I+1, I ), 1, ZERO, W( 1, I ), 1 )
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CALL SGEMV( 'No transpose', N-I, I-1, -ONE, A( I+1, 1 ),
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$ LDA, W( 1, I ), 1, ONE, W( I+1, I ), 1 )
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CALL SGEMV( 'Transpose', N-I, I-1, ONE, A( I+1, 1 ), LDA,
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$ A( I+1, I ), 1, ZERO, W( 1, I ), 1 )
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CALL SGEMV( 'No transpose', N-I, I-1, -ONE, W( I+1, 1 ),
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$ LDW, W( 1, I ), 1, ONE, W( I+1, I ), 1 )
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CALL SSCAL( N-I, TAU( I ), W( I+1, I ), 1 )
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ALPHA = -HALF*TAU( I )*SDOT( N-I, W( I+1, I ), 1,
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$ A( I+1, I ), 1 )
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CALL SAXPY( N-I, ALPHA, A( I+1, I ), 1, W( I+1, I ), 1 )
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END IF
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*
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20 CONTINUE
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END IF
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*
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RETURN
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*
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* End of SLATRD
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*
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END
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