PDDBTRSV(l)

💖 PDDBTRSV(l) — bestie edition 💖

okay so we're doing LAPACK routine, version 1.5, and this one is literally called PDDBTRSV — she's a Fortran subroutine, she's giving linear algebra, she's that girl. sit down, grab a strawberry 🍓, we're solving systems.

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✨ NAME ✨

PDDBTRSV — solves a banded triangular system of linear equations, specifically:

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

periodt. that's the whole job. 🎀

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🦋 SYNOPSIS 🦋

SUBROUTINE PDDBTRSV( UPLO, TRANS, N, BWL, BWU, NRHS, A, JA, DESCA, B,
                     IB, DESCB, AF, LAF, WORK, LWORK, INFO )

    CHARACTER        TRANS, UPLO

    INTEGER          BWL, BWU, IB, INFO, JA, LAF, LWORK, N, NRHS

    INTEGER          DESCA( * ), DESCB( * )

    DOUBLE PRECISION A( * ), AF( * ), B( * ), WORK( * )

look at her. seventeen arguments. she's high maintenance but she delivers. 💅

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🌸 PURPOSE 🌸

PDDBTRSV solves a banded triangular system of linear equations, and you get to pick which flavour:

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

or

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

where A(1:N, JA:JA+N-1) is a banded triangular matrix factor produced by the Gaussian elimination code PD@(dom_pre)BTRF, and it's stored in A(1:N,JA:JA+N-1) and AF.

the matrix sitting in A(1:N, JA:JA+N-1) is either upper or lower triangular, depending on UPLO — you decide, bestie, it's your call. and whether we solve A(1:N, JA:JA+N-1) or A(1:N, JA:JA+N-1)^T is dictated by the user through the parameter TRANS. main character energy: the user is always in charge. 👑

🚨 WAIT. Real talk. Routine PDDBTRF MUST be called first. Like, not optional. Not "if you feel like it". MUST. if you skip it, you're on your own and nobody's coming to save you. 😭

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that's the whole page, no cap — every arg, every *, every capital letter, untouched and accounted for 💖

this was LAPACK routine, version 1.5, dated 12 May 1997 — yeah she's vintage, she's an icon, she's been slaying since before you were born 🎀✨