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# Manual Reference Pages  -  PCPOSV (l)

### NAME

PCPOSV - compute the solution to a complex system of linear equations sub( A ) * X = sub( B ),

Synopsis
Purpose
Arguments

### SYNOPSIS

 SUBROUTINE PCPOSV( UPLO, N, NRHS, A, IA, JA, DESCA, B, IB, JB, DESCB, INFO ) CHARACTER UPLO INTEGER IA, IB, INFO, JA, JB, N, NRHS INTEGER DESCA( * ), DESCB( * ) COMPLEX A( * ), B( * )

### PURPOSE

PCPOSV computes the solution to a complex system of linear equations sub( A ) * X = sub( B ), where sub( A ) denotes A(IA:IA+N-1,JA:JA+N-1) and is an N-by-N hermitian distributed positive definite matrix and X and sub( B ) denoting B(IB:IB+N-1,JB:JB+NRHS-1) are N-by-NRHS distributed matrices.

The Cholesky decomposition is used to factor sub( A ) as

sub( A ) = U**H * U, if UPLO = ’U’, or

sub( A ) = L * L**H, if UPLO = ’L’,

where U is an upper triangular matrix and L is a lower triangular matrix. The factored form of sub( A ) is then used to solve the system of equations.

Notes
=====

Each global data object is described by an associated description vector. This vector stores the information required to establish the mapping between an object element and its corresponding process and memory location.

Let A be a generic term for any 2D block cyclicly distributed array. Such a global array has an associated description vector DESCA. In the following comments, the character _ should be read as "of the global array".

NOTATION STORED IN EXPLANATION
--------------- -------------- -------------------------------------- DTYPE_A(global) DESCA( DTYPE_ )The descriptor type. In this case,
DTYPE_A = 1.
CTXT_A (global) DESCA( CTXT_ ) The BLACS context handle, indicating
the BLACS process grid A is distribu-
ted over. The context itself is glo-
bal, but the handle (the integer
value) may vary.
M_A (global) DESCA( M_ ) The number of rows in the global
array A.
N_A (global) DESCA( N_ ) The number of columns in the global
array A.
MB_A (global) DESCA( MB_ ) The blocking factor used to distribute
the rows of the array.
NB_A (global) DESCA( NB_ ) The blocking factor used to distribute
the columns of the array.
RSRC_A (global) DESCA( RSRC_ ) The process row over which the first
row of the array A is distributed. CSRC_A (global) DESCA( CSRC_ ) The process column over which the
first column of the array A is
distributed.
LLD_A (local) DESCA( LLD_ ) The leading dimension of the local
array. LLD_A >= MAX(1,LOCr(M_A)).

Let K be the number of rows or columns of a distributed matrix, and assume that its process grid has dimension p x q.
LOCr( K ) denotes the number of elements of K that a process would receive if K were distributed over the p processes of its process column.
Similarly, LOCc( K ) denotes the number of elements of K that a process would receive if K were distributed over the q processes of its process row.
The values of LOCr() and LOCc() may be determined via a call to the ScaLAPACK tool function, NUMROC:
LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ). An upper bound for these quantities may be computed by:
LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A

This routine requires square block decomposition ( MB_A = NB_A ).

### ARGUMENTS

 UPLO (global input) CHARACTER = ’U’: Upper triangle of sub( A ) is stored; = ’L’: Lower triangle of sub( A ) is stored. N (global input) INTEGER The number of rows and columns to be operated on, i.e. the order of the distributed submatrix sub( A ). N >= 0. NRHS (global input) INTEGER The number of right hand sides, i.e., the number of columns of the distributed submatrix sub( B ). NRHS >= 0. A (local input/local output) COMPLEX pointer into the local memory to an array of dimension (LLD_A, LOCc(JA+N-1)). On entry, this array contains the local pieces of the N-by-N symmetric distributed matrix sub( A ) to be factored. If UPLO = ’U’, the leading N-by-N upper triangular part of sub( A ) contains the upper triangular part of the matrix, and its strictly lower triangular part is not referenced. If UPLO = ’L’, the leading N-by-N lower triangular part of sub( A ) contains the lower triangular part of the distribu- ted matrix, and its strictly upper triangular part is not referenced. On exit, if INFO = 0, this array contains the local pieces of the factor U or L from the Cholesky factori- zation sub( A ) = U**H*U or L*L**H. IA (global input) INTEGER The row index in the global array A indicating the first row of sub( A ). JA (global input) INTEGER The column index in the global array A indicating the first column of sub( A ). DESCA (global and local input) INTEGER array of dimension DLEN_. The array descriptor for the distributed matrix A. B (local input/local output) COMPLEX pointer into the local memory to an array of dimension (LLD_B,LOC(JB+NRHS-1)). On entry, the local pieces of the right hand sides distribu- ted matrix sub( B ). On exit, if INFO = 0, sub( B ) is over- written with the solution distributed matrix X. IB (global input) INTEGER The row index in the global array B indicating the first row of sub( B ). JB (global input) INTEGER The column index in the global array B indicating the first column of sub( B ). DESCB (global and local input) INTEGER array of dimension DLEN_. The array descriptor for the distributed matrix B. INFO (global output) INTEGER = 0: successful exit < 0: If the i-th argument is an array and the j-entry had an illegal value, then INFO = -(i*100+j), if the i-th argument is a scalar and had an illegal value, then INFO = -i. > 0: If INFO = K, the leading minor of order K, A(IA:IA+K-1,JA:JA+K-1) is not positive definite, and the factorization could not be completed, and the solution has not been computed.
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 ScaLAPACK version 1.7 PCPOSV (l) 13 August 2001

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