zcposv (l)  Linux Manuals
zcposv: computes the solution to a complex system of linear equations A * X = B,
NAME
ZCPOSV  computes the solution to a complex system of linear equations A * X = B,SYNOPSIS
 SUBROUTINE ZCPOSV(
 UPLO, N, NRHS, A, LDA, B, LDB, X, LDX, WORK,
 + SWORK, RWORK, ITER, INFO )
 CHARACTER UPLO
 INTEGER INFO, ITER, LDA, LDB, LDX, N, NRHS
 DOUBLE PRECISION RWORK( * )
 COMPLEX SWORK( * )
 COMPLEX*16 A( LDA, * ), B( LDB, * ), WORK( N, * ),
 + X( LDX, * )
PURPOSE
ZCPOSV computes the solution to a complex system of linear equationsA
ZCPOSV first attempts to factorize the matrix in COMPLEX and use this factorization within an iterative refinement procedure to produce a solution with COMPLEX*16 normwise backward error quality (see below). If the approach fails the method switches to a COMPLEX*16 factorization and solve.
The iterative refinement is not going to be a winning strategy if the ratio COMPLEX performance over COMPLEX*16 performance is too small. A reasonable strategy should take the number of righthand sides and the size of the matrix into account. This might be done with a call to ILAENV in the future. Up to now, we always try iterative refinement.
The iterative refinement process is stopped if
or for all the RHS we have:
where
refinement process
respectively.
ARGUMENTS
 UPLO (input) CHARACTER

= aqUaq: Upper triangle of A is stored;
= aqLaq: 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 or input/ouptut) COMPLEX*16 array,
 dimension (LDA,N) On entry, the Hermitian matrix A. If UPLO = aqUaq, the leading NbyN 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 = aqLaq, the leading NbyN 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. Note that the imaginary parts of the diagonal elements need not be set and are assumed to be zero. On exit, if iterative refinement has been successfully used (INFO.EQ.0 and ITER.GE.0, see description below), then A is unchanged, if double precision factorization has been used (INFO.EQ.0 and ITER.LT.0, see description below), then the array A contains the factor U or L from the Cholesky factorization A = U**H*U or A = L*L**H.
 LDA (input) INTEGER
 The leading dimension of the array A. LDA >= max(1,N).
 B (input) COMPLEX*16 array, dimension (LDB,NRHS)
 The NbyNRHS right hand side matrix B.
 LDB (input) INTEGER
 The leading dimension of the array B. LDB >= max(1,N).
 X (output) COMPLEX*16 array, dimension (LDX,NRHS)
 If INFO = 0, the NbyNRHS solution matrix X.
 LDX (input) INTEGER
 The leading dimension of the array X. LDX >= max(1,N).
 WORK (workspace) COMPLEX*16 array, dimension (N*NRHS)
 This array is used to hold the residual vectors.
 SWORK (workspace) COMPLEX array, dimension (N*(N+NRHS))
 This array is used to use the single precision matrix and the righthand sides or solutions in single precision.
 RWORK (workspace) DOUBLE PRECISION array, dimension (N)
 ITER (output) INTEGER

< 0: iterative refinement has failed, COMPLEX*16
factorization has been performed
1 : the routine fell back to full precision for
implementation or machinespecific reasons
2 : narrowing the precision induced an overflow,
the routine fell back to full precision
3 : failure of CPOTRF
31: stop the iterative refinement after the 30th iterations > 0: iterative refinement has been sucessfully used. Returns the number of iterations  INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
> 0: if INFO = i, the leading minor of order i of (COMPLEX*16) A is not positive definite, so the factorization could not be completed, and the solution has not been computed. =========