PCPTTRS(l)

💖 PCPTTRS 💖

okay so bestie, welcome to the PCPTTRS page — it's a LAPACK routine, and it's lowkey doing the lord's work: solving linear systems. no cap. let's get into it 🫶

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✨ NAME ✨

PCPTTRS</purl> — solve a system of linear equations

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

like. that's the whole vibe. she solves. 💅

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🌸 SYNOPSIS 🌸

SUBROUTINE PCPTTRS( UPLO, N, NRHS, D, E, JA, DESCA, B, IB, DESCB, AF,
                    LAF, WORK, LWORK, INFO )

    CHARACTER       UPLO

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

    INTEGER         DESCA( * ), DESCB( * )

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

    REAL            D( * )

look at all those arguments. she's got range. 👑

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🎀 PURPOSE 🎀

so PCPTTRS solves a system of linear equations

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

where A(1:N, JA:JA+N-1) is the matrix that was used to produce the factors stored in A(1:N,JA:JA+N-1) and AF by PCPTTRF.

and like, A(1:N, JA:JA+N-1) is an N-by-N complex tridiagonal symmetric positive definite distributed matrix. yes she's all of those things at once, she contains multitudes 💜

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

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

or

L*D L'

as computed by PCPTTRF.

🚨 bestie. real talk. routine PCPTTRF MUST be called first. like, that's not optional, that's not a suggestion, that's the whole prerequisite. do not skip her. 😭

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and that's the tea, fr fr — no notes, she ate, periodt 💖✨

this was PCPTTRS, a LAPACK routine, version 1.5, dated 12 May 1997 🌸🦋 bye bestie, go call PCPTTRF first 🫶💅