      SUBROUTINE SSYMV ( UPLO, N, ALPHA, A, LDA, X, INCX,
     $                   BETA, Y, INCY )
*     .. Scalar Arguments ..
      REAL               ALPHA, BETA
      INTEGER            INCX, INCY, LDA, N
      CHARACTER*1        UPLO
*     .. Array Arguments ..
      REAL               A( LDA, * ), X( * ), Y( * )
*     ..
*
*  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
*  ==========
*
*  UPLO   - 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.
*
*           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  - REAL            .
*           On entry, ALPHA specifies the scalar alpha.
*           Unchanged on exit.
*
*  A      - REAL             array of 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.
*           Unchanged on exit.
*
*  LDA    - 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 ).
*           Unchanged on exit.
*
*  X      - REAL             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   - REAL            .
*           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      - REAL             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 ..
      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
*     ..
*     .. 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( 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 .
*
      END
