SRC/zgges3.f(3)

🌸 SRC/zgges3.f 🌸

okay so this is from the Library Functions Manual, and it's literally the hype doc for zgges3 — a LAPACK subroutine that computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices (blocked algorithm). no cap, she's iconic. 💅✨

💖 NAME 💖

SRC/zgges3.f — that's the name, bestie. Simple, cute, done. 🎀

✨ SYNOPSIS ✨

🧚 Functions/Subroutines 🧚

• subroutine zgges3 — yes bestie, she's a subroutine. The whole squad of arguments is below:

subroutine zgges3 (jobvsl, jobvsr, sort, selctg, n, a, lda, b, ldb,
    sdim, alpha, beta, vsl, ldvsl, vsr, ldvsr, work, lwork, rwork,
    bwork, info)

ZGGES3 computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices (blocked algorithm) — periodt. 🔥

🔥 Function/Subroutine Documentation 🔥

Here's the full signature, bestie. Copy it exactly, don't improvise:

subroutine zgges3 (character jobvsl, character jobvsr, character sort,
    external selctg, integer n, complex*16, dimension( lda, * ) a, integer
    lda, complex*16, dimension( ldb, * ) b, integer ldb, integer sdim,
    complex*16, dimension( * ) alpha, complex*16, dimension( * ) beta,
    complex*16, dimension( ldvsl, * ) vsl, integer ldvsl, complex*16,
    dimension( ldvsr, * ) vsr, integer ldvsr, complex*16, dimension( * )
    work, integer lwork, double precision, dimension( * ) rwork, logical,
    dimension( * ) bwork, integer info)

ZGGES3 computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices (blocked algorithm) 💖

🎀 Purpose 🎀

Okay bestie, here's the tea. ZGGES3 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. 💫

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). 🦋

ngl, if only the generalized eigenvalues are needed, use the driver ZGGEV instead — it's faster. 💨

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*16 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 bestie: 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*16 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.
• LDA — LDA is INTEGER. The leading dimension of A. LDA >= max(1,N).
• B — B is COMPLEX*16 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.
• 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*16 array, dimension (N). The shared exit deets live with BETA, bestie. 💖
• BETA — BETA is COMPLEX*16 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 ZGGES3. 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). 💀

• VSL — VSL is COMPLEX*16 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*16 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*16 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.
  • 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.
• RWORK — RWORK is DOUBLE PRECISION 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 ZLAQZ0
    • =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 ZTGSEN.

👑 Author 👑

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

Definition at line 267 of file zgges3.f.

🌈 Author 🌈

Generated automatically by Doxygen for LAPACK from the source code. yup, that's the origin story. 💖

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That's the whole manual, bestie. Footer tea: LAPACK Version 3.12.0, page SRC/zgges3.f(3) — that's the source/version vibe, no separate date in the footer. Slay, stay blocked-algorithm iconic, and remember to read the INFO codes before you panic. 💅👑✨