/home/abui...RC/zgges.f(3)

📦 version 3.12.0, in opensuse-leap 16.0

💖 NAME 💖

Okay so this is the Library Functions Manual page for /home/abui...RC/zgges.f(3), and the actual NAME is this absolutely iconic giant path, bestie:

/home/abuild/rpmbuild/BUILD/lapack-static-3.12.0-build/lapack-3.12.0/SRC/zgges.f

No cap, that's the whole name. It's giving very long path energy, and we respect it. 🫶

✨ SYNOPSIS ✨

🦋 Functions/Subroutines 🦋

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

ZGGES computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices. Periodt. 💅

🧚 Function/Subroutine Documentation 🧚

subroutine ZGGES (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)

ZGGES computes the eigenvalues, the Schur form, and, optionally, the matrix of Schur vectors for GE matrices. Yes bestie, it said it twice — it's main character energy. ✨

🌟 Purpose 🌟

Okay so here's the tea: ZGGES 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 a unitary basis for the corresponding left and right eigenspaces (deflating subspaces). Iconic behavior. 🌈

(If only the generalized eigenvalues are needed, use the driver ZGGEV instead, which is faster.) Ngl, that's the speedrun route. 🔥

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. Lowkey philosophical, but make it linear algebra. 💀

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. That's the vibe check for Schur form. ✅

💅 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.

It 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.

⚠️ Important 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).

🎀 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 ZGGES. The BETA(j) will be non-negative real.

⚠️ Note bestie: 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). So like, respect the norms. 🫡

🎀 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. It's giving "go big or go home" workspace energy. 💅

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 ZHGEQZ
    • =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 zgges.f.

🌈 Author (Doxygen edition) 🌈

Generated automatically by Doxygen for LAPACK from the source code.

And that's the cute little sign-off, bestie: LAPACK Version 3.12.0, page /home/abui...RC/zgges.f(3) — mwah 💋✨

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