ssymv()

subroutine ssymv	(	character	uplo,
		integer	n,
		real	alpha,
		real, dimension(lda,*)	a,
		integer	lda,
		real, dimension(*)	x,
		integer	incx,
		real	beta,
		real, dimension(*)	y,
		integer	incy
	)

SSYMV

Purpose:

 SSYMV  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.

Parameters

[in]	UPLO	UPLO is CHARACTER*1 On entry, UPLO specifies whether the upper or lower triangular part of the array A is to be referenced as follows: UPLO = 'U' or 'u' Only the upper triangular part of A is to be referenced. UPLO = 'L' or 'l' Only the lower triangular part of A is to be referenced.
[in]	N	N is INTEGER On entry, N specifies the order of the matrix A. N must be at least zero.
[in]	ALPHA	ALPHA is REAL On entry, ALPHA specifies the scalar alpha.
[in]	A	A is REAL array, dimension ( LDA, N ) Before entry with UPLO = 'U' or 'u', the leading n by n upper triangular part of the array A must contain the upper triangular part of the symmetric matrix and the strictly lower triangular part of A is not referenced. Before entry with UPLO = 'L' or 'l', the leading n by n lower triangular part of the array A must contain the lower triangular part of the symmetric matrix and the strictly upper triangular part of A is not referenced.
[in]	LDA	LDA is INTEGER On entry, LDA specifies the first dimension of A as declared in the calling (sub) program. LDA must be at least max( 1, n ).
[in]	X	X is REAL array, dimension at least ( 1 + ( n - 1 )*abs( INCX ) ). Before entry, the incremented array X must contain the n element vector x.
[in]	INCX	INCX is INTEGER On entry, INCX specifies the increment for the elements of X. INCX must not be zero.
[in]	BETA	BETA is REAL On entry, BETA specifies the scalar beta. When BETA is supplied as zero then Y need not be set on input.
[in,out]	Y	Y is REAL array, 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.
[in]	INCY	INCY is INTEGER On entry, INCY specifies the increment for the elements of Y. INCY must not be zero.

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Further Details:

  Level 2 Blas routine.
  The vector and matrix arguments are not referenced when N = 0, or M = 0

  -- 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.

Definition at line 151 of file ssymv.f.

*
*  -- Reference BLAS level2 routine --
*  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      REAL ALPHA,BETA
      INTEGER INCX,INCY,LDA,N
      CHARACTER UPLO
*     ..
*     .. Array Arguments ..
      REAL A(LDA,*),X(*),Y(*)
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL ONE,ZERO
      parameter(one=1.0e+0,zero=0.0e+0)
*     ..
*     .. Local Scalars ..
      REAL TEMP1,TEMP2
      INTEGER I,INFO,IX,IY,J,JX,JY,KX,KY
*     ..
*     .. External Functions ..
      LOGICAL LSAME
      EXTERNAL lsame
*     ..
*     .. External Subroutines ..
      EXTERNAL xerbla
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC max
*     ..
*
*     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 (lda.LT.max(1,n)) THEN
          info = 5
      ELSE IF (incx.EQ.0) THEN
          info = 7
      ELSE IF (incy.EQ.0) THEN
          info = 10
      END IF
      IF (info.NE.0) THEN
          CALL xerbla('SSYMV ',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 A are
*     accessed sequentially with one pass through the triangular part
*     of A.
*
*     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
      IF (lsame(uplo,'U')) THEN
*
*        Form  y  when A is stored in upper triangle.
*
          IF ((incx.EQ.1) .AND. (incy.EQ.1)) THEN
              DO 60 j = 1,n
                  temp1 = alpha*x(j)
                  temp2 = zero
                  DO 50 i = 1,j - 1
                      y(i) = y(i) + temp1*a(i,j)
                      temp2 = temp2 + a(i,j)*x(i)
   50             CONTINUE
                  y(j) = y(j) + temp1*a(j,j) + alpha*temp2
   60         CONTINUE
          ELSE
              jx = kx
              jy = ky
              DO 80 j = 1,n
                  temp1 = alpha*x(jx)
                  temp2 = zero
                  ix = kx
                  iy = ky
                  DO 70 i = 1,j - 1
                      y(iy) = y(iy) + temp1*a(i,j)
                      temp2 = temp2 + a(i,j)*x(ix)
                      ix = ix + incx
                      iy = iy + incy
   70             CONTINUE
                  y(jy) = y(jy) + temp1*a(j,j) + alpha*temp2
                  jx = jx + incx
                  jy = jy + incy
   80         CONTINUE
          END IF
      ELSE
*
*        Form  y  when A is stored in 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*a(j,j)
                  DO 90 i = j + 1,n
                      y(i) = y(i) + temp1*a(i,j)
                      temp2 = temp2 + a(i,j)*x(i)
   90             CONTINUE
                  y(j) = y(j) + alpha*temp2
  100         CONTINUE
          ELSE
              jx = kx
              jy = ky
              DO 120 j = 1,n
                  temp1 = alpha*x(jx)
                  temp2 = zero
                  y(jy) = y(jy) + temp1*a(j,j)
                  ix = jx
                  iy = jy
                  DO 110 i = j + 1,n
                      ix = ix + incx
                      iy = iy + incy
                      y(iy) = y(iy) + temp1*a(i,j)
                      temp2 = temp2 + a(i,j)*x(ix)
  110             CONTINUE
                  y(jy) = y(jy) + alpha*temp2
                  jx = jx + incx
                  jy = jy + incy
  120         CONTINUE
          END IF
      END IF
*
      RETURN
*
*     End of SSYMV
*

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