PZGERFS(l)

📦 version 1.5, in ubuntu jammy

💖 PZGERFS — the LAPACK glow-up page 💖

okay bestie, so this is PZGERFS, a LAPACK routine, and yes it's from the l manual section (that's the parallel/l-series, iykyk). It's version 1.5, and it is so much more chill than its name suggests. Sit down, hydrate, let's go. 🫶

✨ NAME ✨

PZGERFS improves the computed solution to a system of linear equations, and hands you error bounds plus backward error estimates for those solutions. That's it, that's the job. 💅

🦋 SYNOPSIS 🦋

SUBROUTINE PZGERFS( TRANS, N, NRHS, A, IA, JA, DESCA, AF, IAF, JAF,
                    DESCAF, IPIV, B, IB, JB, DESCB, X, IX, JX, DESCX,
                    FERR, BERR, WORK, LWORK, RWORK, LRWORK, INFO )

    CHARACTER       TRANS

    INTEGER         IA, IAF, IB, IX, INFO, JA, JAF, JB, JX, LRWORK,
                    LWORK, N, NRHS

    INTEGER         DESCA( * ), DESCAF( * ), DESCB( * ), DESCX( * ),
                    IPIV( * )

    DOUBLE          PRECISION BERR( * ), FERR( * ), RWORK( * )

    COMPLEX*16      A( * ), AF( * ), B( * ), WORK( * ), X( * )

🌸 PURPOSE 🌸

PZGERFS improves the computed solution to a system of linear equations and provides error bounds and backward error estimates for the solutions. iconic behaviour honestly 💖

💜 Notes 💜

Every 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".

DTYPE_A (global) — stored in DESCA( DTYPE_ ): the descriptor type. In this case, DTYPE_A = 1.
CTXT_A (global) — stored in DESCA( CTXT_ ): the BLACS context handle, indicating the BLACS process grid A is distributed over. The context itself is global, but the handle (the integer value) may vary.
M_A (global) — stored in DESCA( M_ ): the number of rows in the global array A.
N_A (global) — stored in DESCA( N_ ): the number of columns in the global array A.
MB_A (global) — stored in DESCA( MB_ ): the blocking factor used to distribute the rows of the array.
NB_A (global) — stored in DESCA( NB_ ): the blocking factor used to distribute the columns of the array.
RSRC_A (global) — stored in DESCA( RSRC_ ): the process row over which the first row of the array A is distributed.
CSRC_A (global) — stored in DESCA( CSRC_ ): the process column over which the first column of the array A is distributed.
LLD_A (local) — stored in 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

In the following comments, sub( A ), sub( X ) and sub( B ) denote respectively A(IA:IA+N-1,JA:JA+N-1), X(IX:IX+N-1,JX:JX+NRHS-1) and B(IB:IB+N-1,JB:JB+NRHS-1). 🎀

👑 ARGUMENTS 👑

TRANS (global input) CHARACTER1* — specifies the form of the system of equations.

= 'N': sub( A ) * sub( X ) = sub( B )          (No transpose)
= 'T': sub( A )**T * sub( X ) = sub( B )          (Transpose)
= 'C': sub( A )**H * sub( X ) = sub( B ) (Conjugate transpose)

N (global input) INTEGER — the order of the matrix sub( A ). N >= 0.
NRHS (global input) INTEGER — the number of right hand sides, i.e., the number of columns of the matrices sub( B ) and sub( X ). NRHS >= 0.
A (local input) COMPLEX16 pointer into the local memory to an array of local dimension (LLD_A,LOCc(JA+N-1)).* — this array contains the local pieces of the distributed matrix sub( A ).
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.
AF (local input) COMPLEX16 pointer into the local memory to an array of local dimension (LLD_AF,LOCc(JA+N-1)).* — this array contains the local pieces of the distributed factors of the matrix sub( A ) = P * L * U as computed by PZGETRF.
IAF (global input) INTEGER — the row index in the global array AF indicating the first row of sub( AF ).
JAF (global input) INTEGER — the column index in the global array AF indicating the first column of sub( AF ).
DESCAF (global and local input) INTEGER array of dimension DLEN_. — the array descriptor for the distributed matrix AF.
IPIV (local input) INTEGER array of dimension LOCr(M_AF)+MB_AF. — this array contains the pivoting information as computed by PZGETRF. IPIV(i) -> The global row local row i was swapped with. This array is tied to the distributed matrix A.
B (local input) COMPLEX16 pointer into the local memory to an array of local dimension (LLD_B,LOCc(JB+NRHS-1)).* — this array contains the local pieces of the distributed matrix of right hand sides sub( B ).
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.
X (local input and output) COMPLEX16 pointer into the local memory to an array of local dimension (LLD_X,LOCc(JX+NRHS-1)).* — on entry, this array contains the local pieces of the distributed matrix solution sub( X ). On exit, the improved solution vectors. ✨
IX (global input) INTEGER — the row index in the global array X indicating the first row of sub( X ).
JX (global input) INTEGER — the column index in the global array X indicating the first column of sub( X ).
DESCX (global and local input) INTEGER array of dimension DLEN_. — the array descriptor for the distributed matrix X.
FERR (local output) DOUBLE PRECISION array of local dimension LOCc(JB+NRHS-1). — the estimated forward error bound for each solution vector of sub( X ). If XTRUE is the true solution corresponding to sub( X ), FERR is an estimated upper bound for the magnitude of the largest element in (sub( X ) - XTRUE) divided by the magnitude of the largest element in sub( X ). The estimate is as reliable as the estimate for RCOND, and is almost always a slight overestimate of the true error. This array is tied to the distributed matrix X.
BERR (local output) DOUBLE PRECISION array of local dimension LOCc(JB+NRHS-1). — the componentwise relative backward error of each solution vector (i.e., the smallest relative change in any entry of sub( A ) or sub( B ) that makes sub( X ) an exact solution). This array is tied to the distributed matrix X.
WORK (local workspace/local output) COMPLEX16 array, dimension (LWORK)* — on exit, WORK(1) returns the minimal and optimal LWORK.
LWORK (local or global input) INTEGER — the dimension of the array WORK. LWORK is local input and must be at least

LWORK >= 2*LOCr( N + MOD(IA-1,MB_A) )

If LWORK = -1, then LWORK is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. Each of these values is returned in the first entry of the corresponding work array, and no error message is issued by PXERBLA.

RWORK (local workspace/local output) DOUBLE PRECISION array, dimension (LRWORK) — on exit, RWORK(1) returns the minimal and optimal LRWORK.
LRWORK (local or global input) INTEGER — the dimension of the array RWORK. LRWORK is local input and must be at least

LRWORK >= LOCr( N + MOD(IB-1,MB_B) ).

If LRWORK = -1, then LRWORK is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. Each of these values is returned in the first entry of the corresponding work array, and no error message is issued by PXERBLA.

INFO (global output) INTEGER — the vibe check report:

= 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.

🌈 PARAMETERS 🌈

ITMAX is the maximum number of steps of iterative refinement. 🧚

💫 Notes 💫

This routine temporarily returns when N <= 1. (not me getting out of bed for one single element 😭)

The distributed submatrices op( A ) and op( AF ) (respectively sub( X ) and sub( B )) should be distributed the same way on the same processes. These conditions ensure that sub( A ) and sub( AF ) (resp. sub( X ) and sub( B )) are "perfectly" aligned. 🫶

Moreover, this routine requires the distributed submatrices sub( A ), sub( AF ), sub( X ), and sub( B ) to be aligned on a block boundary, i.e., if f(x,y) = MOD( x-1, y ):

f( IA, DESCA( MB_ ) ) = f( JA, DESCA( NB_ ) ) = 0,
f( IAF, DESCAF( MB_ ) ) = f( JAF, DESCAF( NB_ ) ) = 0,
f( IB, DESCB( MB_ ) ) = f( JB, DESCB( NB_ ) ) = 0, and
f( IX, DESCX( MB_ ) ) = f( JX, DESCX( NB_ ) ) = 0.

bestie this alignment thing is NOT optional. if your blocks aren't lined up it is over for you. 💀

okay that's the whole page, you ate it, you understood it, main character energy fr fr. This was LAPACK version 1.5, dated 12 May 1997 — yes, she's vintage, she's timeless, she's a classic. 🎀✨👑

📦 where this page comes from

ubuntu jammy: scalapack-doc 1.5-11
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-14: a language model's rewrite of the original, under its terms