*> \brief \b DSPTRD * * =========== DOCUMENTATION =========== * * Online html documentation available at * http://www.netlib.org/lapack/explore-html/ * *> \htmlonly *> Download DSPTRD + dependencies *> *> [TGZ] *> *> [ZIP] *> *> [TXT] *> \endhtmlonly * * Definition: * =========== * * SUBROUTINE DSPTRD( UPLO, N, AP, D, E, TAU, INFO ) * * .. Scalar Arguments .. * CHARACTER UPLO * INTEGER INFO, N * .. * .. Array Arguments .. * DOUBLE PRECISION AP( * ), D( * ), E( * ), TAU( * ) * .. * * *> \par Purpose: * ============= *> *> \verbatim *> *> DSPTRD reduces a real symmetric matrix A stored in packed form to *> symmetric tridiagonal form T by an orthogonal similarity *> transformation: Q**T * A * Q = T. *> \endverbatim * * Arguments: * ========== * *> \param[in] UPLO *> \verbatim *> UPLO is CHARACTER*1 *> = 'U': Upper triangle of A is stored; *> = 'L': Lower triangle of A is stored. *> \endverbatim *> *> \param[in] N *> \verbatim *> N is INTEGER *> The order of the matrix A. N >= 0. *> \endverbatim *> *> \param[in,out] AP *> \verbatim *> AP is DOUBLE PRECISION array, dimension (N*(N+1)/2) *> On entry, the upper or lower triangle of the symmetric matrix *> A, packed columnwise in a linear array. The j-th column of A *> is stored in the array AP as follows: *> if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j; *> if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n. *> On exit, if UPLO = 'U', the diagonal and first superdiagonal *> of A are overwritten by the corresponding elements of the *> tridiagonal matrix T, and the elements above the first *> superdiagonal, with the array TAU, represent the orthogonal *> matrix Q as a product of elementary reflectors; if UPLO *> = 'L', the diagonal and first subdiagonal of A are over- *> written by the corresponding elements of the tridiagonal *> matrix T, and the elements below the first subdiagonal, with *> the array TAU, represent the orthogonal matrix Q as a product *> of elementary reflectors. See Further Details. *> \endverbatim *> *> \param[out] D *> \verbatim *> D is DOUBLE PRECISION array, dimension (N) *> The diagonal elements of the tridiagonal matrix T: *> D(i) = A(i,i). *> \endverbatim *> *> \param[out] E *> \verbatim *> E is DOUBLE PRECISION array, dimension (N-1) *> The off-diagonal elements of the tridiagonal matrix T: *> E(i) = A(i,i+1) if UPLO = 'U', E(i) = A(i+1,i) if UPLO = 'L'. *> \endverbatim *> *> \param[out] TAU *> \verbatim *> TAU is DOUBLE PRECISION array, dimension (N-1) *> The scalar factors of the elementary reflectors (see Further *> Details). *> \endverbatim *> *> \param[out] INFO *> \verbatim *> INFO is INTEGER *> = 0: successful exit *> < 0: if INFO = -i, the i-th argument had an illegal value *> \endverbatim * * Authors: * ======== * *> \author Univ. of Tennessee *> \author Univ. of California Berkeley *> \author Univ. of Colorado Denver *> \author NAG Ltd. * *> \date December 2016 * *> \ingroup doubleOTHERcomputational * *> \par Further Details: * ===================== *> *> \verbatim *> *> If UPLO = 'U', the matrix Q is represented as a product of elementary *> reflectors *> *> Q = H(n-1) . . . H(2) H(1). *> *> Each H(i) has the form *> *> H(i) = I - tau * v * v**T *> *> where tau is a real scalar, and v is a real vector with *> v(i+1:n) = 0 and v(i) = 1; v(1:i-1) is stored on exit in AP, *> overwriting A(1:i-1,i+1), and tau is stored in TAU(i). *> *> If UPLO = 'L', the matrix Q is represented as a product of elementary *> reflectors *> *> Q = H(1) H(2) . . . H(n-1). *> *> Each H(i) has the form *> *> H(i) = I - tau * v * v**T *> *> where tau is a real scalar, and v is a real vector with *> v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in AP, *> overwriting A(i+2:n,i), and tau is stored in TAU(i). *> \endverbatim *> * ===================================================================== SUBROUTINE DSPTRD( UPLO, N, AP, D, E, TAU, INFO ) * * -- LAPACK computational routine (version 3.7.0) -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * December 2016 * * .. Scalar Arguments .. CHARACTER UPLO INTEGER INFO, N * .. * .. Array Arguments .. DOUBLE PRECISION AP( * ), D( * ), E( * ), TAU( * ) * .. * * ===================================================================== * * .. Parameters .. DOUBLE PRECISION ONE, ZERO, HALF PARAMETER ( ONE = 1.0D0, ZERO = 0.0D0, \$ HALF = 1.0D0 / 2.0D0 ) * .. * .. Local Scalars .. LOGICAL UPPER INTEGER I, I1, I1I1, II DOUBLE PRECISION ALPHA, TAUI * .. * .. External Subroutines .. EXTERNAL DAXPY, DLARFG, DSPMV, DSPR2, XERBLA * .. * .. External Functions .. LOGICAL LSAME DOUBLE PRECISION DDOT EXTERNAL LSAME, DDOT * .. * .. Executable Statements .. * * Test the input parameters * INFO = 0 UPPER = LSAME( UPLO, 'U' ) IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN INFO = -1 ELSE IF( N.LT.0 ) THEN INFO = -2 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'DSPTRD', -INFO ) RETURN END IF * * Quick return if possible * IF( N.LE.0 ) \$ RETURN * IF( UPPER ) THEN * * Reduce the upper triangle of A. * I1 is the index in AP of A(1,I+1). * I1 = N*( N-1 ) / 2 + 1 DO 10 I = N - 1, 1, -1 * * Generate elementary reflector H(i) = I - tau * v * v**T * to annihilate A(1:i-1,i+1) * CALL DLARFG( I, AP( I1+I-1 ), AP( I1 ), 1, TAUI ) E( I ) = AP( I1+I-1 ) * IF( TAUI.NE.ZERO ) THEN * * Apply H(i) from both sides to A(1:i,1:i) * AP( I1+I-1 ) = ONE * * Compute y := tau * A * v storing y in TAU(1:i) * CALL DSPMV( UPLO, I, TAUI, AP, AP( I1 ), 1, ZERO, TAU, \$ 1 ) * * Compute w := y - 1/2 * tau * (y**T *v) * v * ALPHA = -HALF*TAUI*DDOT( I, TAU, 1, AP( I1 ), 1 ) CALL DAXPY( I, ALPHA, AP( I1 ), 1, TAU, 1 ) * * Apply the transformation as a rank-2 update: * A := A - v * w**T - w * v**T * CALL DSPR2( UPLO, I, -ONE, AP( I1 ), 1, TAU, 1, AP ) * AP( I1+I-1 ) = E( I ) END IF D( I+1 ) = AP( I1+I ) TAU( I ) = TAUI I1 = I1 - I 10 CONTINUE D( 1 ) = AP( 1 ) ELSE * * Reduce the lower triangle of A. II is the index in AP of * A(i,i) and I1I1 is the index of A(i+1,i+1). * II = 1 DO 20 I = 1, N - 1 I1I1 = II + N - I + 1 * * Generate elementary reflector H(i) = I - tau * v * v**T * to annihilate A(i+2:n,i) * CALL DLARFG( N-I, AP( II+1 ), AP( II+2 ), 1, TAUI ) E( I ) = AP( II+1 ) * IF( TAUI.NE.ZERO ) THEN * * Apply H(i) from both sides to A(i+1:n,i+1:n) * AP( II+1 ) = ONE * * Compute y := tau * A * v storing y in TAU(i:n-1) * CALL DSPMV( UPLO, N-I, TAUI, AP( I1I1 ), AP( II+1 ), 1, \$ ZERO, TAU( I ), 1 ) * * Compute w := y - 1/2 * tau * (y**T *v) * v * ALPHA = -HALF*TAUI*DDOT( N-I, TAU( I ), 1, AP( II+1 ), \$ 1 ) CALL DAXPY( N-I, ALPHA, AP( II+1 ), 1, TAU( I ), 1 ) * * Apply the transformation as a rank-2 update: * A := A - v * w**T - w * v**T * CALL DSPR2( UPLO, N-I, -ONE, AP( II+1 ), 1, TAU( I ), 1, \$ AP( I1I1 ) ) * AP( II+1 ) = E( I ) END IF D( I ) = AP( II ) TAU( I ) = TAUI II = I1I1 20 CONTINUE D( N ) = AP( II ) END IF * RETURN * * End of DSPTRD * END