ssptrd.f

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*> \brief \b SSPTRD
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
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*
*  Definition:
*  ===========
*
*       SUBROUTINE SSPTRD( UPLO, N, AP, D, E, TAU, INFO )
*
*       .. Scalar Arguments ..
*       CHARACTER          UPLO
*       INTEGER            INFO, N
*       ..
*       .. Array Arguments ..
*       REAL               AP( * ), D( * ), E( * ), TAU( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SSPTRD 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 REAL 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 REAL array, dimension (N)
*>          The diagonal elements of the tridiagonal matrix T:
*>          D(i) = A(i,i).
*> \endverbatim
*>
*> \param[out] E
*> \verbatim
*>          E is REAL 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 REAL 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 realOTHERcomputational
*
*> \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 ssptrd( 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 ..
      REAL               AP( * ), D( * ), E( * ), TAU( * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ONE, ZERO, HALF
      parameter( one = 1.0, zero = 0.0, half = 1.0 / 2.0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            UPPER
      INTEGER            I, I1, I1I1, II
      REAL               ALPHA, TAUI
*     ..
*     .. External Subroutines ..
      EXTERNAL           saxpy, slarfg, sspmv, sspr2, xerbla
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      REAL               SDOT
      EXTERNAL           lsame, sdot
*     ..
*     .. 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( 'SSPTRD', -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 slarfg( 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 sspmv( uplo, i, taui, ap, ap( i1 ), 1, zero, tau,
     $                     1 )
*
*              Compute  w := y - 1/2 * tau * (y**T *v) * v
*
               alpha = -half*taui*sdot( i, tau, 1, ap( i1 ), 1 )
               CALL saxpy( i, alpha, ap( i1 ), 1, tau, 1 )
*
*              Apply the transformation as a rank-2 update:
*                 A := A - v * w**T - w * v**T
*
               CALL sspr2( 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 slarfg( 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 sspmv( 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*sdot( n-i, tau( i ), 1, ap( ii+1 ),
     $                 1 )
               CALL saxpy( 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 sspr2( 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 SSPTRD
*
      END