PZGGRQF(l)

📦 version 1.5, in ubuntu jammy

💖 PZGGRQF 💖

okay bestie, sit down, hydrate, because we're reading a page straight out of the l manual (that's the LAPACK/ScaLAPACK section, iykyk) and it is lowkey a banger. this routine is a whole main character and it's about to do algebra on your distributed matrices. no cap. 🫶

🌸 NAME 🌸

PZGGRQF — computes a generalized RQ factorization of an M-by-N matrix sub( A ) = A(IA:IA+M-1,JA:JA+N-1). that's it. that's the job. slay. 💅

✨ SYNOPSIS ✨

SUBROUTINE PZGGRQF( M, P, N, A, IA, JA, DESCA, TAUA, B, IB, JB, DESCB,
                    TAUB, WORK, LWORK, INFO )

    INTEGER         IA, IB, INFO, JA, JB, LWORK, M, N, P

    INTEGER         DESCA( * ), DESCB( * )

    COMPLEX*16      A( * ), B( * ), TAUA( * ), TAUB( * ), WORK( * )

look at all those arguments. iconic. every single one of them matters, so don't skip the ARGUMENTS section, bestie, I'm begging. 🙈

🎀 PURPOSE 🎀

so PZGGRQF computes a generalized RQ factorization of an M-by-N matrix sub( A ) = A(IA:IA+M-1,JA:JA+N-1) and a P-by-N matrix sub( B ) = B(IB:IB+P-1,JB:JB+N-1):

            sub( A ) = R*Q,        sub( B ) = Z*T*Q,

where Q is an N-by-N unitary matrix, Z is a P-by-P unitary matrix, and R and T assume one of these forms:

if M <= N,  R = ( 0  R12 ) M,   or if M > N,  R = ( R11 ) M-N,
                 N-M  M                           ( R21 ) N
                                                    N

where R12 or R21 is upper triangular, and

if P >= N,  T = ( T11 ) N  ,   or if P < N,  T = ( T11  T12 ) P,
                (  0  ) P-N                         P   N-P
                   N

where T11 is upper triangular.

and here's the juicy part 💖 — in particular, if sub( B ) is square and nonsingular, the GRQ factorization of sub( A ) and sub( B ) implicitly gives the RQ factorization of sub( A )*inv( sub( B ) ):

             sub( A )*inv( sub( B ) ) = (R*inv(T))*Z'

where inv( sub( B ) ) denotes the inverse of the matrix sub( B ), and Z' denotes the conjugate transpose of matrix Z. the math is mathing, periodt. ⭐

📝 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. it's giving ✨admin✨ but for matrices.

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) — 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)).

okay so now: 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

👑 ARGUMENTS 👑

M (global input) INTEGER — the number of rows to be operated on i.e the number of rows of the distributed submatrix sub( A ). M >= 0.
P (global input) INTEGER — the number of rows to be operated on i.e the number of rows of the distributed submatrix sub( B ). P >= 0.
N (global input) INTEGER — the number of columns to be operated on i.e the number of columns of the distributed submatrices sub( A ) and sub( B ). N >= 0.
A (local input/local output) COMPLEX*16 pointer into the local memory to an array of dimension (LLD_A, LOCc(JA+N-1)). on entry, the local pieces of the M-by-N distributed matrix sub( A ) which is to be factored. on exit, if M <= N, the upper triangle of A( IA:IA+M-1, JA+N-M:JA+N-1 ) contains the M by M upper triangular matrix R; if M >= N, the elements on and above the (M-N)-th subdiagonal contain the M by N upper trapezoidal matrix R; the remaining elements, with the array TAUA, represent the unitary matrix Q as a product of elementary reflectors (see Further Details). 🔥
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.
TAUA (local output) COMPLEX*16, array, dimension LOCr(IA+M-1) — this array contains the scalar factors of the elementary reflectors which represent the unitary matrix Q. TAUA is tied to the distributed matrix A (see Further Details).
B (local input/local output) COMPLEX*16 pointer into the local memory to an array of dimension (LLD_B, LOCc(JB+N-1)). on entry, the local pieces of the P-by-N distributed matrix sub( B ) which is to be factored. on exit, the elements on and above the diagonal of sub( B ) contain the min(P,N) by N upper trapezoidal matrix T (T is upper triangular if P >= N); the elements below the diagonal, with the array TAUB, represent the unitary matrix Z as a product of elementary reflectors (see Further Details).
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.
TAUB (local output) COMPLEX*16, array, dimension LOCc(JB+MIN(P,N)-1). this array contains the scalar factors TAUB of the elementary reflectors which represent the unitary matrix Z. TAUB is tied to the distributed matrix B (see Further Details).
WORK (local workspace/local output) COMPLEX*16 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 >= MAX( MB_A * ( MpA0 + NqA0 + MB_A ), MAX( (MB_A*(MB_A-1))/2, (PpB0 + NqB0)*MB_A ) + MB_A * MB_A, NB_B * ( PpB0 + NqB0 + NB_B ) ),

where

IROFFA = MOD( IA-1, MB_A ), ICOFFA = MOD( JA-1, NB_A ), IAROW  = INDXG2P( IA, MB_A, MYROW, RSRC_A, NPROW ), IACOL  = INDXG2P( JA, NB_A, MYCOL, CSRC_A, NPCOL ), MpA0   = NUMROC( M+IROFFA, MB_A, MYROW, IAROW, NPROW ), NqA0   = NUMROC( N+ICOFFA, NB_A, MYCOL, IACOL, NPCOL ),

IROFFB = MOD( IB-1, MB_B ), ICOFFB = MOD( JB-1, NB_B ), IBROW  = INDXG2P( IB, MB_B, MYROW, RSRC_B, NPROW ), IBCOL  = INDXG2P( JB, NB_B, MYCOL, CSRC_B, NPCOL ), PpB0   = NUMROC( P+IROFFB, MB_B, MYROW, IBROW, NPROW ), NqB0   = NUMROC( N+ICOFFB, NB_B, MYCOL, IBCOL, NPCOL ),

and NUMROC, INDXG2P are ScaLAPACK tool functions; MYROW, MYCOL, NPROW and NPCOL can be determined by calling the subroutine BLACS_GRIDINFO.

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. sneaky but valid, bestie. 😭

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.

🧚 FURTHER DETAILS 🧚

the matrix Q is represented as a product of elementary reflectors

   Q = H(ia)' H(ia+1)' . . . H(ia+k-1)', where k = min(m,n).

each H(i) has the form

   H(i) = I - taua * v * v'

where taua is a complex scalar, and v is a complex vector with v(n-k+i+1:n) = 0 and v(n-k+i) = 1; conjg(v(1:n-k+i-1)) is stored on exit in A(ia+m-k+i-1,ja:ja+n-k+i-2), and taua in TAUA(ia+m-k+i-1).

to form Q explicitly, use ScaLAPACK subroutine PZUNGRQ.
to use Q to update another matrix, use ScaLAPACK subroutine PZUNMRQ.

the matrix Z is represented as a product of elementary reflectors

   Z = H(jb) H(jb+1) . . . H(jb+k-1), where k = min(p,n).

each H(i) has the form

   H(i) = I - taub * v * v'

where taub is a complex scalar, and v is a complex vector with v(1:i-1) = 0 and v(i) = 1; v(i+1:p) is stored on exit in B(ib+i:ib+p-1,jb+i-1), and taub in TAUB(jb+i-1).

to form Z explicitly, use ScaLAPACK subroutine PZUNGQR.
to use Z to update another matrix, use ScaLAPACK subroutine PZUNMQR.

📐 Alignment requirements 📐

the distributed submatrices sub( A ) and sub( B ) must verify some alignment properties, namely the following expression should be true:

( NB_A.EQ.NB_B .AND. ICOFFA.EQ.ICOFFB .AND. IACOL.EQ.IBCOL )

bestie. fr fr. if that expression isn't true, you're not getting a factorization, you're getting a vibe check failure. check your descriptors before you call this. 💀

and that's the whole page, gorgeous. 💜 this was LAPACK routine version 1.5, dated 12 May 1997 — yes she's a vintage icon and she still eats. go forth, factor your matrices, and remember: LWORK = -1 first. mwah. 🎀✨🦋

📦 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