dsytd2.f

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      SUBROUTINE dsytd2( UPLO, N, A, LDA, D, E, TAU, INFO )
*
*  -- LAPACK routine (version 3.0) --
*     Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
*     Courant Institute, Argonne National Lab, and Rice University
*     October 31, 1992
*
*     .. Scalar Arguments ..
      CHARACTER          UPLO
      INTEGER            INFO, LDA, N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   A( LDA, * ), D( * ), E( * ), TAU( * )
*     ..
*
*  Purpose
*  =======
*
*  DSYTD2 reduces a real symmetric matrix A to symmetric tridiagonal
*  form T by an orthogonal similarity transformation: Q' * A * Q = T.
*
*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          Specifies whether the upper or lower triangular part of the
*          symmetric matrix A is stored:
*          = 'U':  Upper triangular
*          = 'L':  Lower triangular
*
*  N       (input) INTEGER
*          The order of the matrix A.  N >= 0.
*
*  A       (input/output) DOUBLE PRECISION array, dimension (LDA,N)
*          On entry, the symmetric matrix A.  If UPLO = 'U', the leading
*          n-by-n upper triangular part of A contains the upper
*          triangular part of the matrix A, and the strictly lower
*          triangular part of A is not referenced.  If UPLO = 'L', the
*          leading n-by-n lower triangular part of A contains the lower
*          triangular part of the matrix A, and the strictly upper
*          triangular part of A is not referenced.
*          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.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.  LDA >= max(1,N).
*
*  D       (output) DOUBLE PRECISION array, dimension (N)
*          The diagonal elements of the tridiagonal matrix T:
*          D(i) = A(i,i).
*
*  E       (output) 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'.
*
*  TAU     (output) DOUBLE PRECISION array, dimension (N-1)
*          The scalar factors of the elementary reflectors (see Further
*          Details).
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value.
*
*  Further Details
*  ===============
*
*  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'
*
*  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
*  A(1:i-1,i+1), and tau 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'
*
*  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 A(i+2:n,i),
*  and tau in TAU(i).
*
*  The contents of A on exit are illustrated by the following examples
*  with n = 5:
*
*  if UPLO = 'U':                       if UPLO = 'L':
*
*    (  d   e   v2  v3  v4 )              (  d                  )
*    (      d   e   v3  v4 )              (  e   d              )
*    (          d   e   v4 )              (  v1  e   d          )
*    (              d   e  )              (  v1  v2  e   d      )
*    (                  d  )              (  v1  v2  v3  e   d  )
*
*  where d and e denote diagonal and off-diagonal elements of T, and vi
*  denotes an element of the vector defining H(i).
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ONE, ZERO, HALF
      parameter( one = 1.0d0, zero = 0.0d0,
     $                   half = 1.0d0 / 2.0d0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            UPPER
      INTEGER            I
      DOUBLE PRECISION   ALPHA, TAUI
*     ..
*     .. External Subroutines ..
      EXTERNAL           daxpy, dlarfg, dsymv, dsyr2, xerbla
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      DOUBLE PRECISION   DDOT
      EXTERNAL           lsame, ddot
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max, min
*     ..
*     .. 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
      ELSE IF( lda.LT.max( 1, n ) ) THEN
         info = -4
      END IF
      IF( info.NE.0 ) THEN
         CALL xerbla( 'DSYTD2', -info )
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( n.LE.0 )
     $   RETURN
*
      IF( upper ) THEN
*
*        Reduce the upper triangle of A
*
         DO 10 i = n - 1, 1, -1
*
*           Generate elementary reflector H(i) = I - tau * v * v'
*           to annihilate A(1:i-1,i+1)
*
            CALL dlarfg( i, a( i, i+1 ), a( 1, i+1 ), 1, taui )
            e( i ) = a( i, i+1 )
*
            IF( taui.NE.zero ) THEN
*
*              Apply H(i) from both sides to A(1:i,1:i)
*
               a( i, i+1 ) = one
*
*              Compute  x := tau * A * v  storing x in TAU(1:i)
*
               CALL dsymv( uplo, i, taui, a, lda, a( 1, i+1 ), 1, zero,
     $                     tau, 1 )
*
*              Compute  w := x - 1/2 * tau * (x'*v) * v
*
               alpha = -half*taui*ddot( i, tau, 1, a( 1, i+1 ), 1 )
               CALL daxpy( i, alpha, a( 1, i+1 ), 1, tau, 1 )
*
*              Apply the transformation as a rank-2 update:
*                 A := A - v * w' - w * v'
*
               CALL dsyr2( uplo, i, -one, a( 1, i+1 ), 1, tau, 1, a,
     $                     lda )
*
               a( i, i+1 ) = e( i )
            END IF
            d( i+1 ) = a( i+1, i+1 )
            tau( i ) = taui
   10    CONTINUE
         d( 1 ) = a( 1, 1 )
      ELSE
*
*        Reduce the lower triangle of A
*
         DO 20 i = 1, n - 1
*
*           Generate elementary reflector H(i) = I - tau * v * v'
*           to annihilate A(i+2:n,i)
*
            CALL dlarfg( n-i, a( i+1, i ), a( min( i+2, n ), i ), 1,
     $                   taui )
            e( i ) = a( i+1, i )
*
            IF( taui.NE.zero ) THEN
*
*              Apply H(i) from both sides to A(i+1:n,i+1:n)
*
               a( i+1, i ) = one
*
*              Compute  x := tau * A * v  storing y in TAU(i:n-1)
*
               CALL dsymv( uplo, n-i, taui, a( i+1, i+1 ), lda,
     $                     a( i+1, i ), 1, zero, tau( i ), 1 )
*
*              Compute  w := x - 1/2 * tau * (x'*v) * v
*
               alpha = -half*taui*ddot( n-i, tau( i ), 1, a( i+1, i ),
     $                 1 )
               CALL daxpy( n-i, alpha, a( i+1, i ), 1, tau( i ), 1 )
*
*              Apply the transformation as a rank-2 update:
*                 A := A - v * w' - w * v'
*
               CALL dsyr2( uplo, n-i, -one, a( i+1, i ), 1, tau( i ), 1,
     $                     a( i+1, i+1 ), lda )
*
               a( i+1, i ) = e( i )
            END IF
            d( i ) = a( i, i )
            tau( i ) = taui
   20    CONTINUE
         d( n ) = a( n, n )
      END IF
*
      RETURN
*
*     End of DSYTD2
*
      END