PSLATRD(l)

📦 version 1.5, in ubuntu jammy

🎀 PSLATRD 🎀

ok so before anything else: this is PSLATRD, a LAPACK auxiliary routine, and we're living in version 1.5 era, bestie. it's giving parallel linear algebra realness. let's get into it 💅

✨ NAME ✨

PSLATRD — reduce NB rows and columns of a real symmetric distributed matrix sub( A ) = A(IA:IA+N-1,JA:JA+N-1) to symmetric tridiagonal form by an orthogonal similarity transformation Q' * sub( A ) * Q.

🦋 SYNOPSIS 🦋

SUBROUTINE PSLATRD( UPLO, N, NB, A, IA, JA, DESCA, D, E, TAU, W, IW, JW,
                    DESCW, WORK )

    CHARACTER       UPLO

    INTEGER         IA, IW, JA, JW, N, NB

    INTEGER         DESCA( * ), DESCW( * )

    REAL            A( * ), D( * ), E( * ), TAU( * ), W( * ), WORK( * )

💖 PURPOSE 💖

so what does this icon actually DO? PSLATRD reduces NB rows and columns of a real symmetric distributed matrix sub( A ) = A(IA:IA+N-1,JA:JA+N-1) to symmetric tridiagonal form using an orthogonal similarity transformation Q' * sub( A ) * Q, and it hands back the matrices V and W that you need in order to apply that transformation to the unreduced part of sub( A ). no cap. 🔥

the tea on which bits get touched:

if UPLO = 'U', PSLATRD reduces the last NB rows and columns of a matrix, of which the upper triangle is supplied;
if UPLO = 'L', PSLATRD reduces the first NB rows and columns of a matrix, of which the lower triangle is supplied.

and she's not the main character of this story — she's an auxiliary routine, called by PSSYTRD. supporting role but still iconic. 🫶

🌸 Notes 🌸

ok this part is important, it's the whole descriptor-vector lore. 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 👑

UPLO (global input) CHARACTER — specifies whether the upper or lower triangular part of the symmetric matrix sub( A ) is stored:
- = 'U': upper triangular
- = 'L': lower triangular
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.
NB (global input) INTEGER — the number of rows and columns to be reduced.
A (local input/local output) REAL 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 symmetric distributed matrix sub( A ). 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 matrix, and its strictly upper triangular part is not referenced. on exit, if UPLO = 'U', the last NB columns have been reduced to tridiagonal form, with the diagonal elements overwriting the diagonal elements of sub( A ); the elements above the diagonal with the array TAU, represent the orthogonal matrix Q as a product of elementary reflectors. if UPLO = 'L', the first NB columns have been reduced to tridiagonal form, with the diagonal elements overwriting the diagonal elements of sub( A ); the elements below the diagonal with the array TAU, represent the orthogonal 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.
D (local output) REAL array, dimension LOCc(JA+N-1) — the diagonal elements of the tridiagonal matrix T: D(i) = A(i,i). D is tied to the distributed matrix A.
E (local output) REAL array, dimension LOCc(JA+N-1) if UPLO = 'U', LOCc(JA+N-2) otherwise. the off-diagonal elements of the tridiagonal matrix T: E(i) = A(i,i+1) if UPLO = 'U', E(i) = A(i+1,i) if UPLO = 'L'. E is tied to the distributed matrix A.
TAU (local output) REAL, array, dimension LOCc(JA+N-1). this array contains the scalar factors TAU of the elementary reflectors. TAU is tied to the distributed matrix A.
W (local output) REAL pointer into the local memory to an array of dimension (LLD_W,NB_W). this array contains the local pieces of the N-by-NB_W matrix W required to update the unreduced part of sub( A ).
IW (global input) INTEGER — the row index in the global array W indicating the first row of sub( W ).
JW (global input) INTEGER — the column index in the global array W indicating the first column of sub( W ).
DESCW (global and local input) INTEGER array of dimension DLEN_ — the array descriptor for the distributed matrix W.
WORK (local workspace) REAL array, dimension (NB_A).

🌈 FURTHER DETAILS 🌈

if UPLO = 'U', the matrix Q is represented as a product of elementary reflectors

Q = H(n) H(n-1) . . . H(n-nb+1).

each H(i) has the form

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

where tau is a real scalar, and v is a real vector with v(i:n) = 0 and v(i-1) = 1; v(1:i-1) is stored on exit in A(ia:ia+i-2,ja+i), and tau in TAU(ja+i-1).

if UPLO = 'L', the matrix Q is represented as a product of elementary reflectors

Q = H(1) H(2) . . . H(nb).

each H(i) has the form

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

where tau is a real scalar, and v is a real vector with v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in A(ia+i+1:ia+n-1,ja+i-1), and tau in TAU(ja+i-1).

the elements of the vectors v together form the N-by-NB matrix V which is needed, with W, to apply the transformation to the unreduced part of the matrix, using a symmetric rank-2k update of the form: sub( A ) := sub( A ) - V*W' - W*V'.

the contents of A on exit are illustrated by the following examples with n = 5 and nb = 2:

if UPLO = 'U':                       if UPLO = 'L':

  (  a   a   a   v4  v5 )              (  d                  )
  (      a   a   v4  v5 )              (  1   d              )
  (          a   1   v5 )              (  v1  1   a          )
  (              d   1  )              (  v1  v2  a   a      )
  (                  d  )              (  v1  v2  a   a   a  )

where d denotes a diagonal element of the reduced matrix, a denotes an element of the original matrix that is unchanged, and vi denotes an element of the vector defining H(i).

ok that's the whole page bestie, nothing skipped, every single flag and index accounted for 💯 go forth and tridiagonalize. 🦋✨

PSLATRD(l) — this one's from LAPACK, version 1.5, dated 12 May 1997. vintage but she still ate. 💅🎀

📦 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