path: root/SRC/dsptrd.f

*> \brief \b DSPTRD
*
*  =========== DOCUMENTATION ===========
*
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*            http://www.netlib.org/lapack/explore-html/
*
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*
*  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 November 2011
*
*> \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 3.4.0) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2011
*
*     .. 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