SRC/dtfsm.f(3)

📦

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💖 NAME 💖

SRC/dtfsm.f — yes bestie, that's the whole name, and we're in the Library Functions Manual for LAPACK's DTFSM realness ✨

✨ SYNOPSIS ✨

Functions/Subroutines

subroutine dtfsm (transr, side, uplo, trans, diag, m, n, alpha, a, b, ldb) — DTFSM solves a matrix equation (one operand is a triangular matrix in RFP format).

🎀 Function/Subroutine Documentation 🎀

subroutine dtfsm (character transr, character side, character uplo,
    character trans, character diag, integer m, integer n, double precision
    alpha, double precision, dimension( 0: * ) a, double precision,
    dimension( 0: ldb-1, 0: * ) b, integer ldb)

DTFSM solves a matrix equation (one operand is a triangular matrix in RFP format).

💅 Purpose:

Okay so this is a Level 3 BLAS like routine for A in RFP Format.

DTFSM solves the matrix equation

op( A )*X = alpha*B  or  X*op( A ) = alpha*B

where alpha is a scalar, X and B are m by n matrices, A is a unit, or non-unit, upper or lower triangular matrix and op( A ) is one of

op( A ) = A   or   op( A ) = A**T.

A is in Rectangular Full Packed (RFP) Format.

The matrix X is overwritten on B.

🌸 Parameters 🌸

TRANSR — TRANSR is CHARACTER*1
- = 'N': The Normal Form of RFP A is stored;
- = 'T': The Transpose Form of RFP A is stored.
SIDE — SIDE is CHARACTER*1
- On entry, SIDE specifies whether op( A ) appears on the left or right of X as follows:
- SIDE = 'L' or 'l' → op( A )*X = alpha*B.
- SIDE = 'R' or 'r' → X*op( A ) = alpha*B.
- Unchanged on exit.
UPLO — UPLO is CHARACTER*1
- On entry, UPLO specifies whether the RFP matrix A came from an upper or lower triangular matrix as follows:
- UPLO = 'U' or 'u' → RFP A came from an upper triangular matrix
- UPLO = 'L' or 'l' → RFP A came from a lower triangular matrix
- Unchanged on exit.
TRANS — TRANS is CHARACTER*1
- On entry, TRANS specifies the form of op( A ) to be used in the matrix multiplication as follows:
- TRANS = 'N' or 'n' → op( A ) = A.
- TRANS = 'T' or 't' → op( A ) = A'.
- Unchanged on exit.
DIAG — DIAG is CHARACTER*1
- On entry, DIAG specifies whether or not RFP A is unit triangular as follows:
- DIAG = 'U' or 'u' → A is assumed to be unit triangular.
- DIAG = 'N' or 'n' → A is not assumed to be unit triangular.
- Unchanged on exit.
M — M is INTEGER
- On entry, M specifies the number of rows of B. M must be at least zero.
- Unchanged on exit.
N — N is INTEGER
- On entry, N specifies the number of columns of B. N must be at least zero.
- Unchanged on exit.
ALPHA — ALPHA is DOUBLE PRECISION
- On entry, ALPHA specifies the scalar alpha. When alpha is zero then A is not referenced and B need not be set before entry.
- Unchanged on exit.
A — A is DOUBLE PRECISION array, dimension (NT)
- NT = N*(N+1)/2 if SIDE='R' and NT = M*(M+1)/2 otherwise.
- On entry, the matrix A in RFP Format.
- RFP Format is described by TRANSR, UPLO and N as follows:
- If TRANSR='N' then RFP A is (0:N,0:K-1) when N is even; K=N/2. RFP A is (0:N-1,0:K) when N is odd; K=N/2. If TRANSR = 'T' then RFP is the transpose of RFP A as defined when TRANSR = 'N'. The contents of RFP A are defined by UPLO as follows: If UPLO = 'U' the RFP A contains the NT elements of upper packed A either in normal or transpose Format. If UPLO = 'L' the RFP A contains the NT elements of lower packed A either in normal or transpose Format. The LDA of RFP A is (N+1)/2 when TRANSR = 'T'. When TRANSR is 'N' the LDA is N+1 when N is even and is N when is odd. See the Note below for more details. Unchanged on exit.
B — B is DOUBLE PRECISION array, dimension (LDB,N)
- Before entry, the leading m by n part of the array B must contain the right-hand side matrix B, and on exit is overwritten by the solution matrix X.
LDB — LDB is INTEGER
- On entry, LDB specifies the first dimension of B as declared in the calling (sub) program. LDB must be at least max( 1, m ).
- Unchanged on exit.

bestie, LDB must be at least max( 1, m ) — that's not a suggestion, it's the rule 💅

👑 Author 👑

Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

🦋 Further Details 🦋

Okay bestie, RFP format time. First up: RFP when N is even. Example where N = 6.

AP is Upper and AP is Lower:

      AP is Upper             AP is Lower

   00 01 02 03 04 05       00
      11 12 13 14 15       10 11
         22 23 24 25       20 21 22
            33 34 35       30 31 32 33
               44 45       40 41 42 43 44
                  55       50 51 52 53 54 55

Let TRANSR = 'N'. RFP holds AP as follows:

For UPLO = 'U' the upper trapezoid A(0:5,0:2) consists of the last three columns of AP upper. The lower triangle A(4:6,0:2) consists of the transpose of the first three columns of AP upper.
For UPLO = 'L' the lower trapezoid A(1:6,0:2) consists of the first three columns of AP lower. The upper triangle A(0:2,0:2) consists of the transpose of the last three columns of AP lower.

This covers the case N even and TRANSR = 'N'.

        RFP A                   RFP A

        03 04 05                33 43 53
        13 14 15                00 44 54
        23 24 25                10 11 55
        33 34 35                20 21 22
        00 44 45                30 31 32
        01 11 55                40 41 42
        02 12 22                50 51 52

Now let TRANSR = 'T'. RFP A in both UPLO cases is just the transpose of RFP A above. One therefore gets:

           RFP A                   RFP A

     03 13 23 33 00 01 02    33 00 10 20 30 40 50
     04 14 24 34 44 11 12    43 44 11 21 31 41 51
     05 15 25 35 45 55 22    53 54 55 22 32 42 52

We then consider RFP when N is odd. Example where N = 5.

AP is Upper and AP is Lower:

   AP is Upper                 AP is Lower

   00 01 02 03 04              00
      11 12 13 14              10 11
         22 23 24              20 21 22
            33 34              30 31 32 33
               44              40 41 42 43 44

Let TRANSR = 'N'. RFP holds AP as follows:

For UPLO = 'U' the upper trapezoid A(0:4,0:2) consists of the last three columns of AP upper. The lower triangle A(3:4,0:1) consists of the transpose of the first two columns of AP upper.
For UPLO = 'L' the lower trapezoid A(0:4,0:2) consists of the first three columns of AP lower. The upper triangle A(0:1,1:2) consists of the transpose of the last two columns of AP lower.

This covers the case N odd and TRANSR = 'N'.

        RFP A                   RFP A

        02 03 04                00 33 43
        12 13 14                10 11 44
        22 23 24                20 21 22
        00 33 34                30 31 32
        01 11 44                40 41 42

Now let TRANSR = 'T'. RFP A in both UPLO cases is just the transpose of RFP A above. One therefore gets:

           RFP A                   RFP A

     02 12 22 00 01             00 10 20 30 40 50
     03 13 23 33 11             33 11 21 31 41 51
     04 14 24 34 44             43 44 22 32 42 52

Definition at line 276 of file dtfsm.f. — that's the source coordinate, bestie 💅

🌈 Author 🌈

Generated automatically by Doxygen for LAPACK from the source code.

🍓 ok that's the whole page, bestie — footer vibes: LAPACK Version 3.12.0, page SRC/dtfsm.f(3), and the footer didn't give me a date so I'm not making one up. mwah 💋✨

📦 where this page comes from

arch rolling: lapack-doc 3.12.1-2 · custom
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms