SRC/cgges3.f(3)

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💅 SRC/cgges3.f(3) — from the Library Functions Manual

okay bestie, so this is a page straight out of the Library Functions Manual, and it's about SRC/cgges3.f — like, the file itself. no cap, it's a whole document about one Fortran file. let's get into it 🎀

✨ NAME ✨

SRC/cgges3.f — that's it, that's the name. iconic minimalism honestly 🦋

🌸 SYNOPSIS 🌸

Functions/Subroutines

subroutine cgges3 (jobvsl, jobvsr, sort, selctg, n, a, lda, b, ldb,
    sdim, alpha, beta, vsl, ldvsl, vsr, ldvsr, work, lwork, rwork,
    bwork, info)

CGGES3 computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices (blocked algorithm) 🔥

👑 Function/Subroutine Documentation 👑

subroutine cgges3 (character jobvsl, character jobvsr, character sort,
    external selctg, integer n, complex, dimension( lda, * ) a, integer lda,
    complex, dimension( ldb, * ) b, integer ldb, integer sdim, complex,
    dimension( * ) alpha, complex, dimension( * ) beta, complex, dimension(
    ldvsl, * ) vsl, integer ldvsl, complex, dimension( ldvsr, * ) vsr,
    integer ldvsr, complex, dimension( * ) work, integer lwork, real,
    dimension( * ) rwork, logical, dimension( * ) bwork, integer info)

CGGES3 computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices (blocked algorithm) 💜

Purpose 💖

CGGES3 computes for a pair of N-by-N complex nonsymmetric matrices (A,B), the generalized eigenvalues, the generalized complex Schur form (S, T), and optionally left and/or right Schur vectors (VSL and VSR). This gives the generalized Schur factorization

(A,B) = ( (VSL)*S*(VSR)**H, (VSL)*T*(VSR)**H )

where (VSR)**H is the conjugate-transpose of VSR. slay 💅

Optionally, it also orders the eigenvalues so that a selected cluster of eigenvalues appears in the leading diagonal blocks of the upper triangular matrix S and the upper triangular matrix T. The leading columns of VSL and VSR then form an unitary basis for the corresponding left and right eigenspaces (deflating subspaces) ✨

(If only the generalized eigenvalues are needed, use the driver CGGEV instead, which is faster.) — like, genuinely faster, not me being dramatic 😭

A generalized eigenvalue for a pair of matrices (A,B) is a scalar w or a ratio alpha/beta = w, such that A - w*B is singular. It is usually represented as the pair (alpha,beta), as there is a reasonable interpretation for beta=0, and even for both being zero 🫶

A pair of matrices (S,T) is in generalized complex Schur form if S and T are upper triangular and, in addition, the diagonal elements of T are non-negative real numbers ⭐

Parameters 🌈

JOBVSL

JOBVSL is CHARACTER*1

  • = 'N': do not compute the left Schur vectors;
  • = 'V': compute the left Schur vectors.

JOBVSR

JOBVSR is CHARACTER*1

  • = 'N': do not compute the right Schur vectors;
  • = 'V': compute the right Schur vectors.

SORT

SORT is CHARACTER*1. Specifies whether or not to order the eigenvalues on the diagonal of the generalized Schur form.

  • = 'N': Eigenvalues are not ordered;
  • = 'S': Eigenvalues are ordered (see SELCTG).

SELCTG

SELCTG is a LOGICAL FUNCTION of two COMPLEX arguments.

SELCTG must be declared EXTERNAL in the calling subroutine.

If SORT = 'N', SELCTG is not referenced.

If SORT = 'S', SELCTG is used to select eigenvalues to sort to the top left of the Schur form.

An eigenvalue ALPHA(j)/BETA(j) is selected if SELCTG(ALPHA(j),BETA(j)) is true.

Note that a selected complex eigenvalue may no longer satisfy SELCTG(ALPHA(j),BETA(j)) = .TRUE. after ordering, since ordering may change the value of complex eigenvalues (especially if the eigenvalue is ill-conditioned), in this case INFO is set to N+2 (See INFO below). 🫣

N

N is INTEGER. The order of the matrices A, B, VSL, and VSR. N >= 0.

A

A is COMPLEX array, dimension (LDA, N).

On entry, the first of the pair of matrices. On exit, A has been overwritten by its generalized Schur form S. she got rewritten, it's fine 💀

LDA

LDA is INTEGER. The leading dimension of A. LDA >= max(1,N).

B

B is COMPLEX array, dimension (LDB, N).

On entry, the second of the pair of matrices. On exit, B has been overwritten by its generalized Schur form T. same deal, glow-up happened 🤭

LDB

LDB is INTEGER. The leading dimension of B. LDB >= max(1,N).

SDIM

SDIM is INTEGER.

  • If SORT = 'N', SDIM = 0.
  • If SORT = 'S', SDIM = number of eigenvalues (after sorting) for which SELCTG is true.

ALPHA

ALPHA is COMPLEX array, dimension (N).

BETA

BETA is COMPLEX array, dimension (N).

On exit, ALPHA(j)/BETA(j), j=1,...,N, will be the generalized eigenvalues. ALPHA(j), j=1,...,N and BETA(j), j=1,...,N are the diagonals of the complex Schur form (A,B) output by CGGES3. The BETA(j) will be non-negative real 🌸

Note: the quotients ALPHA(j)/BETA(j) may easily over- or underflow, and BETA(j) may even be zero. Thus, the user should avoid naively computing the ratio alpha/beta. However, ALPHA will be always less than and usually comparable with norm(A) in magnitude, and BETA always less than and usually comparable with norm(B). bestie do NOT just divide them, i'm begging 😭

VSL

VSL is COMPLEX array, dimension (LDVSL,N).

If JOBVSL = 'V', VSL will contain the left Schur vectors. Not referenced if JOBVSL = 'N'.

LDVSL

LDVSL is INTEGER. The leading dimension of the matrix VSL. LDVSL >= 1, and if JOBVSL = 'V', LDVSL >= N.

VSR

VSR is COMPLEX array, dimension (LDVSR,N).

If JOBVSR = 'V', VSR will contain the right Schur vectors. Not referenced if JOBVSR = 'N'.

LDVSR

LDVSR is INTEGER. The leading dimension of the matrix VSR. LDVSR >= 1, and if JOBVSR = 'V', LDVSR >= N.

WORK

WORK is COMPLEX array, dimension (MAX(1,LWORK)).

On exit, if INFO = 0, WORK(1) returns the optimal LWORK ✨

LWORK

LWORK is INTEGER. The dimension of the array WORK. LWORK >= MAX(1,2*N). For good performance, LWORK must generally be larger. highkey make it bigger, your future self will thank you 🫶

If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA. sneaky little size-check mode, we love her 💅

RWORK

RWORK is REAL array, dimension (8*N).

BWORK

BWORK is LOGICAL array, dimension (N).

Not referenced if SORT = 'N'.

INFO

INFO is INTEGER.

  • = 0: successful exit 🎉
  • < 0: if INFO = -i, the i-th argument had an illegal value.
  • =1,...,N: The QZ iteration failed. (A,B) are not in Schur form, but ALPHA(j) and BETA(j) should be correct for j=INFO+1,...,N.
  • > N:
    • =N+1: other than QZ iteration failed in CLAQZ0
    • =N+2: after reordering, roundoff changed values of some complex eigenvalues so that leading eigenvalues in the Generalized Schur form no longer satisfy SELCTG=.TRUE. This could also be caused due to scaling.
    • =N+3: reordering failed in CTGSEN.

🧚 Author 🧚

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

Definition at line 267 of file cgges3.f. 📍

💫 Author (again, yes really) 💫

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