DSYSV man page on IRIX

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DSYSV(3F)							     DSYSV(3F)

NAME
     DSYSV - compute the solution to a real system of linear equations	A * X
     = B,

SYNOPSIS
     SUBROUTINE DSYSV( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, WORK, LWORK, INFO
		       )

	 CHARACTER     UPLO

	 INTEGER       INFO, LDA, LDB, LWORK, N, NRHS

	 INTEGER       IPIV( * )

	 DOUBLE	       PRECISION A( LDA, * ), B( LDB, * ), WORK( LWORK )

PURPOSE
     DSYSV computes the solution to a real system of linear equations
	A * X = B, where A is an N-by-N symmetric matrix and X and B are N-
     by-NRHS matrices.

     The diagonal pivoting method is used to factor A as
	A = U * D * U**T,  if UPLO = 'U', or
	A = L * D * L**T,  if UPLO = 'L',
     where U (or L) is a product of permutation and unit upper (lower)
     triangular matrices, and D is symmetric and block diagonal with 1-by-1
     and 2-by-2 diagonal blocks.  The factored form of A is then used to solve
     the system of equations A * X = B.

ARGUMENTS
     UPLO    (input) CHARACTER*1
	     = 'U':  Upper triangle of A is stored;
	     = 'L':  Lower triangle of A is stored.

     N	     (input) INTEGER
	     The number of linear equations, i.e., the order of the matrix A.
	     N >= 0.

     NRHS    (input) INTEGER
	     The number of right hand sides, i.e., the number of columns of
	     the matrix B.  NRHS >= 0.

     A	     (input/output) DOUBLE PRECISION array, dimension (LDA,N)
	     On entry, the symmetric matrix A.	If UPLO = 'U', the leading N-
	     by-N upper triangular part of A contains the upper triangular
	     part of the matrix A, and the strictly lower triangular part of A
	     is not referenced.	 If UPLO = 'L', the leading N-by-N lower
	     triangular part of A contains the lower triangular part of the
	     matrix A, and the strictly upper triangular part of A is not
	     referenced.

									Page 1

DSYSV(3F)							     DSYSV(3F)

	     On exit, if INFO = 0, the block diagonal matrix D and the
	     multipliers used to obtain the factor U or L from the
	     factorization A = U*D*U**T or A = L*D*L**T as computed by DSYTRF.

     LDA     (input) INTEGER
	     The leading dimension of the array A.  LDA >= max(1,N).

     IPIV    (output) INTEGER array, dimension (N)
	     Details of the interchanges and the block structure of D, as
	     determined by DSYTRF.  If IPIV(k) > 0, then rows and columns k
	     and IPIV(k) were interchanged, and D(k,k) is a 1-by-1 diagonal
	     block.  If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0, then rows and
	     columns k-1 and -IPIV(k) were interchanged and D(k-1:k,k-1:k) is
	     a 2-by-2 diagonal block.  If UPLO = 'L' and IPIV(k) = IPIV(k+1) <
	     0, then rows and columns k+1 and -IPIV(k) were interchanged and
	     D(k:k+1,k:k+1) is a 2-by-2 diagonal block.

     B	     (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
	     On entry, the N-by-NRHS right hand side matrix B.	On exit, if
	     INFO = 0, the N-by-NRHS solution matrix X.

     LDB     (input) INTEGER
	     The leading dimension of the array B.  LDB >= max(1,N).

     WORK    (workspace/output) DOUBLE PRECISION array, dimension (LWORK)
	     On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

     LWORK   (input) INTEGER
	     The length of WORK.  LWORK >= 1, and for best performance LWORK
	     >= N*NB, where NB is the optimal blocksize for DSYTRF.

     INFO    (output) INTEGER
	     = 0: successful exit
	     < 0: if INFO = -i, the i-th argument had an illegal value
	     > 0: if INFO = i, D(i,i) is exactly zero.	The factorization has
	     been completed, but the block diagonal matrix D is exactly
	     singular, so the solution could not be computed.

									Page 2

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