DGBTRF(3)

DGBTRF(3)

yo bestie, welcome to DGBTRF(3) from the Library Functions Manual — this one is a whole LAPACK subroutine and she is lowkey iconic 💅✨

🌸 NAME 🌸

/home/abuild/rpmbuild/BUILD/lapack-static-3.12.0-build/lapack-3.12.0/SRC/dgbtrf.f — that's the file path, bestie. That's the whole NAME vibe 🫶

✨ SYNOPSIS ✨

🧚 Functions/Subroutines

subroutine DGBTRF (m, n, kl, ku, ab, ldab, ipiv, info)
    DGBTRF

💖 Function/Subroutine Documentation 💖

subroutine DGBTRF (integer m, integer n, integer kl, integer ku, double
    precision, dimension( ldab, * ) ab, integer ldab, integer, dimension( *
    ) ipiv, integer info)
    DGBTRF

🎀 Purpose 🎀

DGBTRF computes an LU factorization of a real m-by-n band matrix A using partial pivoting with row interchanges. no cap.

This is the blocked version of the algorithm, calling Level 3 BLAS. periodt 🔥

🌈 Parameters 🌈

• M — M is INTEGER; the number of rows of the matrix A. M >= 0.
• N — N is INTEGER; the number of columns of the matrix A. N >= 0.
• KL — KL is INTEGER; the number of subdiagonals within the band of A. KL >= 0.
• KU — KU is INTEGER; the number of superdiagonals within the band of A. KU >= 0.
• AB — AB is DOUBLE PRECISION array, dimension (LDAB,N). On entry, the matrix A in band storage, in rows KL+1 to 2*KL+KU+1; rows 1 to KL of the array need not be set. The j-th column of A is stored in the j-th column of the array AB as follows: AB(kl+ku+1+i-j,j) = A(i,j) for max(1,j-ku)<=i<=min(m,j+kl)

On exit, details of the factorization: U is stored as an upper triangular band matrix with KL+KU superdiagonals in rows 1 to KL+KU+1, and the multipliers used during the factorization are stored in rows KL+KU+2 to 2*KL+KU+1. See below for further details 👀

• LDAB — LDAB is INTEGER; the leading dimension of the array AB. LDAB >= 2*KL+KU+1.
• IPIV — IPIV is INTEGER array, dimension (min(M,N)); the pivot indices; for 1 <= i <= min(M,N), row i of the matrix was interchanged with row IPIV(i).
• INFO — INFO is INTEGER:
  • = 0: successful exit ✨
  • < 0: if INFO = -i, the i-th argument had an illegal value
  • > 0: if INFO = +i, U(i,i) is exactly zero. The factorization has been completed, but the factor U is exactly singular, and division by zero will occur if it is used to solve a system of equations.

bestie, that INFO > 0 case is the “U is exactly singular” warning — division by zero will occur if it is used to solve a system of equations. do not ignore her 💀

👑 Author 👑

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

🦋 Further Details 🦋

The band storage scheme is illustrated by the following example, when M = N = 6, KL = 2, KU = 1:

On entry:                       On exit:

      *    *    *    +    +    +       *    *    *   u14  u25  u36
      *    *    +    +    +    +       *    *   u13  u24  u35  u46
      *   a12  a23  a34  a45  a56      *   u12  u23  u34  u45  u56
     a11  a22  a33  a44  a55  a66     u11  u22  u33  u44  u55  u66
     a21  a32  a43  a54  a65   *      m21  m32  m43  m54  m65   *
     a31  a42  a53  a64   *    *      m31  m42  m53  m64   *    *

Array elements marked * are not used by the routine; elements marked + need not be set on entry, but are required by the routine to store elements of U because of fill-in resulting from the row interchanges. iykyk 🌸

Definition at line 143 of file dgbtrf.f.

👑 Author 👑

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mwah 💋 — signing off with LAPACK Version 3.12.0 and the footer path /home/abui...RC/dgbtrf.f(3). slay 🎀