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dspmv.f

      SUBROUTINE DSPMV ( UPLO, N, ALPHA, AP, X, INCX, BETA, Y, INCY )
*     .. Scalar Arguments ..
      DOUBLE PRECISION   ALPHA, BETA
      INTEGER            INCX, INCY, N
      CHARACTER*1        UPLO
*     .. Array Arguments ..
      DOUBLE PRECISION   AP( * ), X( * ), Y( * )
*     ..
*
*  Purpose
*  =======
*
*  DSPMV  performs the matrix-vector operation
*
*     y := alpha*A*x + beta*y,
*
*  where alpha and beta are scalars, x and y are n element vectors and
*  A is an n by n symmetric matrix, supplied in packed form.
*
*  Parameters
*  ==========
*
*  UPLO   - CHARACTER*1.
*           On entry, UPLO specifies whether the upper or lower
*           triangular part of the matrix A is supplied in the packed
*           array AP as follows:
*
*              UPLO = 'U' or 'u'   The upper triangular part of A is
*                                  supplied in AP.
*
*              UPLO = 'L' or 'l'   The lower triangular part of A is
*                                  supplied in AP.
*
*           Unchanged on exit.
*
*  N      - INTEGER.
*           On entry, N specifies the order of the matrix A.
*           N must be at least zero.
*           Unchanged on exit.
*
*  ALPHA  - DOUBLE PRECISION.
*           On entry, ALPHA specifies the scalar alpha.
*           Unchanged on exit.
*
*  AP     - DOUBLE PRECISION array of DIMENSION at least
*           ( ( n*( n + 1 ) )/2 ).
*           Before entry with UPLO = 'U' or 'u', the array AP must
*           contain the upper triangular part of the symmetric matrix
*           packed sequentially, column by column, so that AP( 1 )
*           contains a( 1, 1 ), AP( 2 ) and AP( 3 ) contain a( 1, 2 )
*           and a( 2, 2 ) respectively, and so on.
*           Before entry with UPLO = 'L' or 'l', the array AP must
*           contain the lower triangular part of the symmetric matrix
*           packed sequentially, column by column, so that AP( 1 )
*           contains a( 1, 1 ), AP( 2 ) and AP( 3 ) contain a( 2, 1 )
*           and a( 3, 1 ) respectively, and so on.
*           Unchanged on exit.
*
*  X      - DOUBLE PRECISION array of dimension at least
*           ( 1 + ( n - 1 )*abs( INCX ) ).
*           Before entry, the incremented array X must contain the n
*           element vector x.
*           Unchanged on exit.
*
*  INCX   - INTEGER.
*           On entry, INCX specifies the increment for the elements of
*           X. INCX must not be zero.
*           Unchanged on exit.
*
*  BETA   - DOUBLE PRECISION.
*           On entry, BETA specifies the scalar beta. When BETA is
*           supplied as zero then Y need not be set on input.
*           Unchanged on exit.
*
*  Y      - DOUBLE PRECISION array of dimension at least
*           ( 1 + ( n - 1 )*abs( INCY ) ).
*           Before entry, the incremented array Y must contain the n
*           element vector y. On exit, Y is overwritten by the updated
*           vector y.
*
*  INCY   - INTEGER.
*           On entry, INCY specifies the increment for the elements of
*           Y. INCY must not be zero.
*           Unchanged on exit.
*
*
*  Level 2 Blas routine.
*
*  -- Written on 22-October-1986.
*     Jack Dongarra, Argonne National Lab.
*     Jeremy Du Croz, Nag Central Office.
*     Sven Hammarling, Nag Central Office.
*     Richard Hanson, Sandia National Labs.
*
*
*     .. Parameters ..
      DOUBLE PRECISION   ONE         , ZERO
      PARAMETER        ( ONE = 1.0D+0, ZERO = 0.0D+0 )
*     .. Local Scalars ..
      DOUBLE PRECISION   TEMP1, TEMP2
      INTEGER            I, INFO, IX, IY, J, JX, JY, K, KK, KX, KY
*     .. External Functions ..
      LOGICAL            LSAME
      EXTERNAL           LSAME
*     .. External Subroutines ..
      EXTERNAL           XERBLA
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      IF     ( .NOT.LSAME( UPLO, 'U' ).AND.
     $         .NOT.LSAME( UPLO, 'L' )      )THEN
         INFO = 1
      ELSE IF( N.LT.0 )THEN
         INFO = 2
      ELSE IF( INCX.EQ.0 )THEN
         INFO = 6
      ELSE IF( INCY.EQ.0 )THEN
         INFO = 9
      END IF
      IF( INFO.NE.0 )THEN
         CALL XERBLA( 'DSPMV ', INFO )
         RETURN
      END IF
*
*     Quick return if possible.
*
      IF( ( N.EQ.0 ).OR.( ( ALPHA.EQ.ZERO ).AND.( BETA.EQ.ONE ) ) )
     $   RETURN
*
*     Set up the start points in  X  and  Y.
*
      IF( INCX.GT.0 )THEN
         KX = 1
      ELSE
         KX = 1 - ( N - 1 )*INCX
      END IF
      IF( INCY.GT.0 )THEN
         KY = 1
      ELSE
         KY = 1 - ( N - 1 )*INCY
      END IF
*
*     Start the operations. In this version the elements of the array AP
*     are accessed sequentially with one pass through AP.
*
*     First form  y := beta*y.
*
      IF( BETA.NE.ONE )THEN
         IF( INCY.EQ.1 )THEN
            IF( BETA.EQ.ZERO )THEN
               DO 10, I = 1, N
                  Y( I ) = ZERO
   10          CONTINUE
            ELSE
               DO 20, I = 1, N
                  Y( I ) = BETA*Y( I )
   20          CONTINUE
            END IF
         ELSE
            IY = KY
            IF( BETA.EQ.ZERO )THEN
               DO 30, I = 1, N
                  Y( IY ) = ZERO
                  IY      = IY   + INCY
   30          CONTINUE
            ELSE
               DO 40, I = 1, N
                  Y( IY ) = BETA*Y( IY )
                  IY      = IY           + INCY
   40          CONTINUE
            END IF
         END IF
      END IF
      IF( ALPHA.EQ.ZERO )
     $   RETURN
      KK = 1
      IF( LSAME( UPLO, 'U' ) )THEN
*
*        Form  y  when AP contains the upper triangle.
*
         IF( ( INCX.EQ.1 ).AND.( INCY.EQ.1 ) )THEN
            DO 60, J = 1, N
               TEMP1 = ALPHA*X( J )
               TEMP2 = ZERO
               K     = KK
               DO 50, I = 1, J - 1
                  Y( I ) = Y( I ) + TEMP1*AP( K )
                  TEMP2  = TEMP2  + AP( K )*X( I )
                  K      = K      + 1
   50          CONTINUE
               Y( J ) = Y( J ) + TEMP1*AP( KK + J - 1 ) + ALPHA*TEMP2
               KK     = KK     + J
   60       CONTINUE
         ELSE
            JX = KX
            JY = KY
            DO 80, J = 1, N
               TEMP1 = ALPHA*X( JX )
               TEMP2 = ZERO
               IX    = KX
               IY    = KY
               DO 70, K = KK, KK + J - 2
                  Y( IY ) = Y( IY ) + TEMP1*AP( K )
                  TEMP2   = TEMP2   + AP( K )*X( IX )
                  IX      = IX      + INCX
                  IY      = IY      + INCY
   70          CONTINUE
               Y( JY ) = Y( JY ) + TEMP1*AP( KK + J - 1 ) + ALPHA*TEMP2
               JX      = JX      + INCX
               JY      = JY      + INCY
               KK      = KK      + J
   80       CONTINUE
         END IF
      ELSE
*
*        Form  y  when AP contains the lower triangle.
*
         IF( ( INCX.EQ.1 ).AND.( INCY.EQ.1 ) )THEN
            DO 100, J = 1, N
               TEMP1  = ALPHA*X( J )
               TEMP2  = ZERO
               Y( J ) = Y( J )       + TEMP1*AP( KK )
               K      = KK           + 1
               DO 90, I = J + 1, N
                  Y( I ) = Y( I ) + TEMP1*AP( K )
                  TEMP2  = TEMP2  + AP( K )*X( I )
                  K      = K      + 1
   90          CONTINUE
               Y( J ) = Y( J ) + ALPHA*TEMP2
               KK     = KK     + ( N - J + 1 )
  100       CONTINUE
         ELSE
            JX = KX
            JY = KY
            DO 120, J = 1, N
               TEMP1   = ALPHA*X( JX )
               TEMP2   = ZERO
               Y( JY ) = Y( JY )       + TEMP1*AP( KK )
               IX      = JX
               IY      = JY
               DO 110, K = KK + 1, KK + N - J
                  IX      = IX      + INCX
                  IY      = IY      + INCY
                  Y( IY ) = Y( IY ) + TEMP1*AP( K )
                  TEMP2   = TEMP2   + AP( K )*X( IX )
  110          CONTINUE
               Y( JY ) = Y( JY ) + ALPHA*TEMP2
               JX      = JX      + INCX
               JY      = JY      + INCY
               KK      = KK      + ( N - J + 1 )
  120       CONTINUE
         END IF
      END IF
*
      RETURN
*
*     End of DSPMV .
*
      END

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