PCDTTRSV(l)

🌸 NAME 🌸

PCDTTRSV — bestie, this one solves a tridiagonal triangular system of linear equations, like:

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

That's the whole vibe. Tridiagonal. Triangular. Solving. Slay. 💅

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✨ SYNOPSIS ✨

SUBROUTINE PCDTTRSV( UPLO, TRANS, N, NRHS, DL, D, DU, JA, DESCA, B, IB,
                     DESCB, AF, LAF, WORK, LWORK, INFO )

    CHARACTER        TRANS, UPLO

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

    INTEGER          DESCA( * ), DESCB( * )

    COMPLEX          AF( * ), B( * ), D( * ), DL( * ), DU( * ), WORK( *
                     )

Look at all those arguments. Not me counting them like a girlboss. 🙈 Every single one of them is real and every single one of them matters, no cap.

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🦋 PURPOSE 🦋

Okay so here's the tea, bestie. PCDTTRSV solves a tridiagonal triangular system of linear equations:

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

or

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

Like, pick your fighter. 🔥

So A(1:N, JA:JA+N-1) is a tridiagonal triangular matrix factor produced by the Gaussian elimination code PC@(dom_pre)TTRF, and it's stored in A(1:N,JA:JA+N-1) and AF. Yes, both. It's giving storage goals. ✨

The matrix chilling in A(1:N, JA:JA+N-1) is either upper or lower triangular, and that's entirely according to UPLO — so, like, UPLO is the one calling the shots there. 💅

And whether you're solving A(1:N, JA:JA+N-1) or its conjugate transpose A(1:N, JA:JA+N-1)^H? That's dictated by the user through the parameter TRANS. You are the main character. You decide. 👑

⚠️ Real talk though, bestie: routine PCDTTRF MUST be called first. Not "should", not "maybe", not "if you feel like it". MUST. It's giving prerequisite, and you cannot skip it. 😭

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LAPACK routine, version 1.5 — dated 12 May 1997, and yes she's still iconic. 🎀 Bye bestie, go call PCDTTRF first! 💖