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      SUBROUTINE SSYR2F ( UPLO, N, ALPHA, X, INCX, Y, INCY, A, LDA )
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*     .. Scalar Arguments ..
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      REAL               ALPHA
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      INTEGER            INCX, INCY, LDA, N
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      CHARACTER*1        UPLO
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*     .. Array Arguments ..
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      REAL               A( LDA, * ), X( * ), Y( * )
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*     ..
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*
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*  Purpose
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*  =======
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*
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*  SSYR2  performs the symmetric rank 2 operation
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*
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*     A := alpha*x*y' + alpha*y*x' + A,
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*
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*  where alpha is a scalar, x and y are n element vectors and A is an n
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*  by n symmetric matrix.
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*
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*  Parameters
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*  ==========
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*
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*  UPLO   - CHARACTER*1.
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*           On entry, UPLO specifies whether the upper or lower
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*           triangular part of the array A is to be referenced as
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*           follows:
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*
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*              UPLO = 'U' or 'u'   Only the upper triangular part of A
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*                                  is to be referenced.
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*
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*              UPLO = 'L' or 'l'   Only the lower triangular part of A
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*                                  is to be referenced.
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*
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*           Unchanged on exit.
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*
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*  N      - INTEGER.
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*           On entry, N specifies the order of the matrix A.
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*           N must be at least zero.
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*           Unchanged on exit.
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*
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*  ALPHA  - REAL            .
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*           On entry, ALPHA specifies the scalar alpha.
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*           Unchanged on exit.
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*
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*  X      - REAL             array of dimension at least
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*           ( 1 + ( n - 1 )*abs( INCX ) ).
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*           Before entry, the incremented array X must contain the n
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*           element vector x.
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*           Unchanged on exit.
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*
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*  INCX   - INTEGER.
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*           On entry, INCX specifies the increment for the elements of
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*           X. INCX must not be zero.
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*           Unchanged on exit.
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*
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*  Y      - REAL             array of dimension at least
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*           ( 1 + ( n - 1 )*abs( INCY ) ).
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*           Before entry, the incremented array Y must contain the n
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*           element vector y.
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*           Unchanged on exit.
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*
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*  INCY   - INTEGER.
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*           On entry, INCY specifies the increment for the elements of
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*           Y. INCY must not be zero.
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*           Unchanged on exit.
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*
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*  A      - REAL             array of DIMENSION ( LDA, n ).
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*           Before entry with  UPLO = 'U' or 'u', the leading n by n
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*           upper triangular part of the array A must contain the upper
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*           triangular part of the symmetric matrix and the strictly
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*           lower triangular part of A is not referenced. On exit, the
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*           upper triangular part of the array A is overwritten by the
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*           upper triangular part of the updated matrix.
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*           Before entry with UPLO = 'L' or 'l', the leading n by n
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*           lower triangular part of the array A must contain the lower
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*           triangular part of the symmetric matrix and the strictly
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*           upper triangular part of A is not referenced. On exit, the
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*           lower triangular part of the array A is overwritten by the
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*           lower triangular part of the updated matrix.
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*
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*  LDA    - INTEGER.
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*           On entry, LDA specifies the first dimension of A as declared
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*           in the calling (sub) program. LDA must be at least
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*           max( 1, n ).
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*           Unchanged on exit.
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*
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*
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*  Level 2 Blas routine.
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*
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*  -- Written on 22-October-1986.
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*     Jack Dongarra, Argonne National Lab.
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*     Jeremy Du Croz, Nag Central Office.
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*     Sven Hammarling, Nag Central Office.
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*     Richard Hanson, Sandia National Labs.
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*
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*
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*     .. Parameters ..
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      REAL               ZERO
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      PARAMETER        ( ZERO = 0.0E+0 )
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*     .. Local Scalars ..
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      REAL               TEMP1, TEMP2
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      INTEGER            I, INFO, IX, IY, J, JX, JY, KX, KY
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*     .. External Functions ..
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      LOGICAL            LSAME
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      EXTERNAL           LSAME
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*     .. External Subroutines ..
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      EXTERNAL           XERBLA
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*     .. Intrinsic Functions ..
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      INTRINSIC          MAX
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*     ..
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*     .. Executable Statements ..
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*
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*     Test the input parameters.
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*
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      INFO = 0
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      IF     ( .NOT.LSAME( UPLO, 'U' ).AND.
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     $         .NOT.LSAME( UPLO, 'L' )      )THEN
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         INFO = 1
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      ELSE IF( N.LT.0 )THEN
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         INFO = 2
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      ELSE IF( INCX.EQ.0 )THEN
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         INFO = 5
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      ELSE IF( INCY.EQ.0 )THEN
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         INFO = 7
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      ELSE IF( LDA.LT.MAX( 1, N ) )THEN
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         INFO = 9
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      END IF
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      IF( INFO.NE.0 )THEN
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         CALL XERBLA( 'SSYR2 ', INFO )
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         RETURN
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      END IF
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*
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*     Quick return if possible.
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*
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      IF( ( N.EQ.0 ).OR.( ALPHA.EQ.ZERO ) )
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     $   RETURN
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*
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*     Set up the start points in X and Y if the increments are not both
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*     unity.
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*
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      IF( ( INCX.NE.1 ).OR.( INCY.NE.1 ) )THEN
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         IF( INCX.GT.0 )THEN
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            KX = 1
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         ELSE
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            KX = 1 - ( N - 1 )*INCX
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         END IF
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         IF( INCY.GT.0 )THEN
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            KY = 1
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         ELSE
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            KY = 1 - ( N - 1 )*INCY
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         END IF
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         JX = KX
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         JY = KY
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      END IF
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*
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*     Start the operations. In this version the elements of A are
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*     accessed sequentially with one pass through the triangular part
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*     of A.
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*
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      IF( LSAME( UPLO, 'U' ) )THEN
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*
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*        Form  A  when A is stored in the upper triangle.
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*
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         IF( ( INCX.EQ.1 ).AND.( INCY.EQ.1 ) )THEN
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            DO 20, J = 1, N
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               IF( ( X( J ).NE.ZERO ).OR.( Y( J ).NE.ZERO ) )THEN
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                  TEMP1 = ALPHA*Y( J )
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                  TEMP2 = ALPHA*X( J )
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                  DO 10, I = 1, J
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                     A( I, J ) = A( I, J ) + X( I )*TEMP1 + Y( I )*TEMP2
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   10             CONTINUE
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               END IF
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   20       CONTINUE
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         ELSE
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            DO 40, J = 1, N
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               IF( ( X( JX ).NE.ZERO ).OR.( Y( JY ).NE.ZERO ) )THEN
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                  TEMP1 = ALPHA*Y( JY )
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                  TEMP2 = ALPHA*X( JX )
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                  IX    = KX
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                  IY    = KY
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                  DO 30, I = 1, J
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                     A( I, J ) = A( I, J ) + X( IX )*TEMP1
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     $                                     + Y( IY )*TEMP2
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                     IX        = IX        + INCX
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                     IY        = IY        + INCY
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   30             CONTINUE
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               END IF
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               JX = JX + INCX
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               JY = JY + INCY
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   40       CONTINUE
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         END IF
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      ELSE
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*
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*        Form  A  when A is stored in the lower triangle.
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*
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         IF( ( INCX.EQ.1 ).AND.( INCY.EQ.1 ) )THEN
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            DO 60, J = 1, N
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               IF( ( X( J ).NE.ZERO ).OR.( Y( J ).NE.ZERO ) )THEN
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                  TEMP1 = ALPHA*Y( J )
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                  TEMP2 = ALPHA*X( J )
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                  DO 50, I = J, N
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                     A( I, J ) = A( I, J ) + X( I )*TEMP1 + Y( I )*TEMP2
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   50             CONTINUE
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               END IF
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   60       CONTINUE
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         ELSE
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            DO 80, J = 1, N
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               IF( ( X( JX ).NE.ZERO ).OR.( Y( JY ).NE.ZERO ) )THEN
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                  TEMP1 = ALPHA*Y( JY )
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                  TEMP2 = ALPHA*X( JX )
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                  IX    = JX
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                  IY    = JY
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                  DO 70, I = J, N
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                     A( I, J ) = A( I, J ) + X( IX )*TEMP1
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     $                                     + Y( IY )*TEMP2
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                     IX        = IX        + INCX
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                     IY        = IY        + INCY
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   70             CONTINUE
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               END IF
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               JX = JX + INCX
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               JY = JY + INCY
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   80       CONTINUE
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         END IF
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      END IF
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*
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      RETURN
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*
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*     End of SSYR2 .
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*
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      END