SUBROUTINE SSYR2F ( UPLO, N, ALPHA, X, INCX, Y, INCY, A, LDA )

*     .. Scalar Arguments ..

      REAL               ALPHA

      INTEGER            INCX, INCY, LDA, N

      CHARACTER*1        UPLO

*     .. Array Arguments ..

      REAL               A( LDA, * ), X( * ), Y( * )

*     ..

*

*  Purpose

*  =======

*

*  SSYR2  performs the symmetric rank 2 operation

*

*     A := alpha*x*y' + alpha*y*x' + A,

*

*  where alpha is a scalar, 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.

*

*  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.

*

*  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.

*           Unchanged on exit.

*

*  INCY   - INTEGER.

*           On entry, INCY specifies the increment for the elements of

*           Y. INCY must not be zero.

*           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. On exit, the

*           upper triangular part of the array A is overwritten by the

*           upper triangular part of the updated matrix.

*           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. On exit, the

*           lower triangular part of the array A is overwritten by the

*           lower triangular part of the updated matrix.

*

*  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.

*

*

*  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               ZERO

      PARAMETER        ( 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( INCX.EQ.0 )THEN

         INFO = 5

      ELSE IF( INCY.EQ.0 )THEN

         INFO = 7

      ELSE IF( LDA.LT.MAX( 1, N ) )THEN

         INFO = 9

      END IF

      IF( INFO.NE.0 )THEN

         CALL XERBLA( 'SSYR2 ', INFO )

         RETURN

      END IF

*

*     Quick return if possible.

*

      IF( ( N.EQ.0 ).OR.( ALPHA.EQ.ZERO ) )

     $   RETURN

*

*     Set up the start points in X and Y if the increments are not both

*     unity.

*

      IF( ( INCX.NE.1 ).OR.( INCY.NE.1 ) )THEN

         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

         JX = KX

         JY = KY

      END IF

*

*     Start the operations. In this version the elements of A are

*     accessed sequentially with one pass through the triangular part

*     of A.

*

      IF( LSAME( UPLO, 'U' ) )THEN

*

*        Form  A  when A is stored in the upper triangle.

*

         IF( ( INCX.EQ.1 ).AND.( INCY.EQ.1 ) )THEN

            DO 20, J = 1, N

               IF( ( X( J ).NE.ZERO ).OR.( Y( J ).NE.ZERO ) )THEN

                  TEMP1 = ALPHA*Y( J )

                  TEMP2 = ALPHA*X( J )

                  DO 10, I = 1, J

                     A( I, J ) = A( I, J ) + X( I )*TEMP1 + Y( I )*TEMP2

   10             CONTINUE

               END IF

   20       CONTINUE

         ELSE

            DO 40, J = 1, N

               IF( ( X( JX ).NE.ZERO ).OR.( Y( JY ).NE.ZERO ) )THEN

                  TEMP1 = ALPHA*Y( JY )

                  TEMP2 = ALPHA*X( JX )

                  IX    = KX

                  IY    = KY

                  DO 30, I = 1, J

                     A( I, J ) = A( I, J ) + X( IX )*TEMP1

     $                                     + Y( IY )*TEMP2

                     IX        = IX        + INCX

                     IY        = IY        + INCY

   30             CONTINUE

               END IF

               JX = JX + INCX

               JY = JY + INCY

   40       CONTINUE

         END IF

      ELSE

*

*        Form  A  when A is stored in the lower triangle.

*

         IF( ( INCX.EQ.1 ).AND.( INCY.EQ.1 ) )THEN

            DO 60, J = 1, N

               IF( ( X( J ).NE.ZERO ).OR.( Y( J ).NE.ZERO ) )THEN

                  TEMP1 = ALPHA*Y( J )

                  TEMP2 = ALPHA*X( J )

                  DO 50, I = J, N

                     A( I, J ) = A( I, J ) + X( I )*TEMP1 + Y( I )*TEMP2

   50             CONTINUE

               END IF

   60       CONTINUE

         ELSE

            DO 80, J = 1, N

               IF( ( X( JX ).NE.ZERO ).OR.( Y( JY ).NE.ZERO ) )THEN

                  TEMP1 = ALPHA*Y( JY )

                  TEMP2 = ALPHA*X( JX )

                  IX    = JX

                  IY    = JY

                  DO 70, I = J, N

                     A( I, J ) = A( I, J ) + X( IX )*TEMP1

     $                                     + Y( IY )*TEMP2

                     IX        = IX        + INCX

                     IY        = IY        + INCY

   70             CONTINUE

               END IF

               JX = JX + INCX

               JY = JY + INCY

   80       CONTINUE

         END IF

      END IF

*

      RETURN

*

*     End of SSYR2 .

*

      END