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RedHat 9 (Linux i386) - man page for dspsv (redhat section l)

DSPSV(l)					)					 DSPSV(l)

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

SYNOPSIS
       SUBROUTINE DSPSV( UPLO, N, NRHS, AP, IPIV, B, LDB, INFO )

	   CHARACTER	 UPLO

	   INTEGER	 INFO, LDB, N, NRHS

	   INTEGER	 IPIV( * )

	   DOUBLE	 PRECISION AP( * ), B( LDB, * )

PURPOSE
       DSPSV  computes the solution to a real system of linear equations A * X = B, where A is an
       N-by-N symmetric matrix stored in packed format 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,	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.

       AP      (input/output) DOUBLE PRECISION array, dimension (N*(N+1)/2)
	       On entry, the upper or lower triangle of the symmetric matrix A, packed columnwise
	       in a linear array.  The j-th column of A is stored in the array AP as follows:  if
	       UPLO  =	'U',  AP(i  +  (j-1)*j/2)  =  A(i,j)  for  1<=i<=j; if UPLO = 'L', AP(i +
	       (j-1)*(2n-j)/2) = A(i,j) for j<=i<=n.  See below for further details.

	       On exit, 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 DSPTRF,
	       stored as a packed triangular matrix in the same storage format as A.

       IPIV    (output) INTEGER array, dimension (N)
	       Details of the interchanges and the block structure of D, as determined by DSPTRF.
	       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).

       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.

FURTHER DETAILS
       The packed storage scheme is illustrated by the following example when N = 4, UPLO = 'U':

       Two-dimensional storage of the symmetric matrix A:

	  a11 a12 a13 a14
	      a22 a23 a24
		  a33 a34     (aij = aji)
		      a44

       Packed storage of the upper triangle of A:

       AP = [ a11, a12, a22, a13, a23, a33, a14, a24, a34, a44 ]

LAPACK version 3.0			   15 June 2000 				 DSPSV(l)


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