PSLARZT(l)

📦 version 1.5, in ubuntu jammy

💅 PSLARZT(l) — LAPACK auxiliary routine, bestie edition 💅

okay so this is PSLARZT, a LAPACK auxiliary routine (from the <pink>l</pink> manual, i.e. the local one), and yes she's still out here running on LAPACK version 1.5 energy. Buckle up, we're forming triangular factors and it's honestly giving math girlboss. ✨

🌸 NAME 🌸

PSLARZT — form the triangular factor T of a real block reflector H of order > n, which is defined as a product of k elementary reflectors as returned by PSTZRZF. no cap, that's literally her whole job. 🫶

🎀 SYNOPSIS 🎀

SUBROUTINE PSLARZT( DIRECT, STOREV, N, K, V, IV, JV, DESCV, TAU, T, WORK
                    )

    CHARACTER       DIRECT, STOREV

    INTEGER         IV, JV, K, N

    INTEGER         DESCV( * )

    REAL            TAU( * ), T( * ), V( * ), WORK( * )

like look at that signature. gorgeous. iconic. 💖

🦋 PURPOSE 🦋

PSLARZT forms the triangular factor T of a real block reflector H of order > n, which is defined as a product of k elementary reflectors as returned by PSTZRZF.

so here's the vibe breakdown:

If DIRECT = 'F', then H = H(1) H(2) . . . H(k) and T is upper triangular;
If DIRECT = 'B', then H = H(k) . . . H(2) H(1) and T is lower triangular.

and for the vectors:

If STOREV = 'C', the vector which defines the elementary reflector H(i) is stored in the i-th column of the array V, and

H  =  I - V * T * V'

If STOREV = 'R', the vector which defines the elementary reflector H(i) is stored in the i-th row of the array V, and

H  =  I - V' * T * V

lowkey important: currently, only STOREV = 'R' and DIRECT = 'B' are supported. don't come for her about the others, she's working on it. 😭

📝 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".

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

👑 ARGUMENTS 👑

DIRECT (global input) CHARACTER — specifies the order in which the elementary reflectors are multiplied to form the block reflector:
- = 'F': H = H(1) H(2) . . . H(k) (Forward, not supported yet)
- = 'B': H = H(k) . . . H(2) H(1) (Backward)
STOREV (global input) CHARACTER — specifies how the vectors which define the elementary reflectors are stored (see also Further Details):
- = 'R': rowwise
N (global input) INTEGER — the number of meaningful entries of the block reflector H. N >= 0. (yes bestie, zero is allowed, we love an inclusive queen 🫶)
K (global input) INTEGER — the order of the triangular factor T (= the number of elementary reflectors). 1 <= K <= MB_V (= NB_V).
V (input/output) REAL pointer into the local memory to an array of local dimension (LOCr(IV+K-1),LOCc(JV+N-1)). The distributed matrix V contains the Householder vectors. See further details. (yes she's both input AND output, she contains multitudes ✨)
IV (global input) INTEGER — the row index in the global array V indicating the first row of sub( V ).
JV (global input) INTEGER — the column index in the global array V indicating the first column of sub( V ).
DESCV (global and local input) INTEGER array of dimension DLEN_. The array descriptor for the distributed matrix V.
TAU (local input) REAL, array, dimension LOCr(IV+K-1) if INCV = M_V, and LOCc(JV+K-1) otherwise. This array contains the Householder scalars related to the Householder vectors. TAU is tied to the distributed matrix V. (they're attached, it's a whole thing 💜)
T (local output) REAL array, dimension (MB_V,MB_V). It contains the k-by-k triangular factor of the block reflector associated with V. T is lower triangular.
WORK (local workspace) REAL array, dimension (K*(K-1)/2)

🌈 FURTHER DETAILS 🌈

The shape of the matrix V and the storage of the vectors which define the H(i) is best illustrated by the following example with n = 5 and k = 3. The elements equal to 1 are not stored; the corresponding array elements are modified but restored on exit. The rest of the array is not used.

DIRECT = 'F' and STOREV = 'C':         DIRECT = 'F' and STOREV = 'R':

                                            ______V_____
       ( v1 v2 v3 )                        /                   ( v1 v2
v3 )                      ( v1 v1 v1 v1 v1 . . . . 1 )
   V = ( v1 v2 v3 )                      ( v2 v2 v2 v2 v2 . . . 1   )
       ( v1 v2 v3 )                      ( v3 v3 v3 v3 v3 . . 1     )
       ( v1 v2 v3 )
          .  .  .
          .  .  .
          1  .  .
             1  .
                1

DIRECT = 'B' and STOREV = 'C':         DIRECT = 'B' and STOREV = 'R':

                                                      ______V_____
          1                                          /
.  1                           ( 1 . . . . v1 v1 v1 v1 v1 )
          .  .  1                        ( . 1 . . . v2 v2 v2 v2 v2 )
          .  .  .                        ( . . 1 . . v3 v3 v3 v3 v3 )
          .  .  .
       ( v1 v2 v3 )
       ( v1 v2 v3 )
   V = ( v1 v2 v3 )
       ( v1 v2 v3 )
       ( v1 v2 v3 )

like, stare at it for a sec. it'll click. trust. 🧚

and that's the whole page, bestie!! this was PSLARZT(l), straight out of LAPACK version 1.5, dated 12 May 1997 — yes she's vintage, yes she still slays. 💖✨🎀 go forth and form those triangular factors. periodt. 💅

📦 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