PCPBTRS(l)

📦 version 1.5, in ubuntu jammy

💖 NAME 💖

PCPBTRS — bestie, this routine solves a system of linear equations

A(1:N, JA:JA+N-1) * X = B(IB:IB+N-1, 1:NRHS)

like that's literally its whole job and it eats every time 💅

✨ SYNOPSIS ✨

SUBROUTINE PCPBTRS( UPLO, N, BW, NRHS, A, JA, DESCA, B, IB, DESCB, AF,
                    LAF, WORK, LWORK, INFO )

    CHARACTER       UPLO

    INTEGER         BW, IB, INFO, JA, LAF, LWORK, N, NRHS

    INTEGER         DESCA( * ), DESCB( * )

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

🦋 PURPOSE 🦋

okay so PCPBTRS solves a system of linear equations

where A(1:N, JA:JA+N-1) is the matrix used to produce the factors stored in A(1:N,JA:JA+N-1) and AF by PCPBTRF 👑

A(1:N, JA:JA+N-1) is an N-by-N complex banded symmetric positive definite distributed matrix with bandwidth BW. no cap, that's a whole lot of adjectives but every single one of them is doing something 🎀

depending on the value of UPLO, A stores either U or L in the equn

A(1:N, JA:JA+N-1) = U'*U

or

L*L'

as computed by PCPBTRF 🌸

⚠️ heads up bestie: routine PCPBTRF MUST be called first. like, no PCPBTRF, no PCPBTRS. it's giving prerequisite, it's giving "do the homework before the exam" 😭

that's the whole page, and honestly? iconic behavior from a banded solver.

this was LAPACK version 1.5, dated 12 May 1997 — yeah she's a vintage queen and she still slays 💖✨🫶

📦 where this page comes from