SRC/ssygvd.f(3)

💖 NAME 💖

okay so this page is SRC/ssygvd.f(3), straight outta the Library Functions Manual, and it's all about the SSYGVD subroutine. bestie, we're getting into eigenvalues and eigenvectors today ✨

---

✨ SYNOPSIS ✨

Functions/Subroutines

subroutine ssygvd (itype, jobz, uplo, n, a, lda, b, ldb, w, work, lwork,
           iwork, liwork, info)
           SSYGVD

---

🦋 Function/Subroutine Documentation 🦋

subroutine ssygvd (integer itype, character jobz, character uplo, integer n,
       real, dimension( lda, * ) a, integer lda, real, dimension( ldb, * ) b,
       integer ldb, real, dimension( * ) w, real, dimension( * ) work, integer
       lwork, integer, dimension( * ) iwork, integer liwork, integer info)
       SSYGVD

🌸 Purpose

SSYGVD computes all the eigenvalues, and optionally, the eigenvectors of a real generalized symmetric-definite eigenproblem, of the form A*x=(lambda)*B*x, A*Bx=(lambda)*x, or B*A*x=(lambda)*x. Here A and B are assumed to be symmetric and B is also positive definite. If eigenvectors are desired, it uses a divide and conquer algorithm. no cap, that's the whole vibe 💅

🎀 Parameters

• ITYPE — ITYPE is INTEGER. It specifies the problem type to be solved:
  • = 1: A*x = (lambda)*B*x
  • = 2: A*B*x = (lambda)*x
  • = 3: B*A*x = (lambda)*x
• JOBZ — JOBZ is CHARACTER*1. It says how much we're computing:
  • 'N': compute eigenvalues only;
  • 'V': compute eigenvalues and eigenvectors.
• UPLO — UPLO is CHARACTER*1. It tells us which triangle is stored:
  • 'U': upper triangles of A and B are stored;
  • 'L': lower triangles of A and B are stored.
• N — N is INTEGER. The order of the matrices A and B. N >= 0.
• A — A is REAL array, dimension (LDA, N). On entry, the symmetric matrix A. If UPLO = 'U', the leading N-by-N upper triangular part of A contains the upper triangular part of the matrix A. If UPLO = 'L', the leading N-by-N lower triangular part of A contains the lower triangular part of the matrix A. On exit, if JOBZ = 'V', then if INFO = 0, A contains the matrix Z of eigenvectors. The eigenvectors are normalized as follows: if ITYPE = 1 or 2, Z**T*B*Z = I; if ITYPE = 3, Z**T*inv(B)*Z = I. If JOBZ = 'N', then on exit the upper triangle (if UPLO='U') or the lower triangle (if UPLO='L') of A, including the diagonal, is destroyed. 😭 don't say I didn't warn you.
• LDA — LDA is INTEGER. The leading dimension of the array A. LDA >= max(1,N).
• B — B is REAL array, dimension (LDB, N). On entry, the symmetric matrix B. If UPLO = 'U', the leading N-by-N upper triangular part of B contains the upper triangular part of the matrix B. If UPLO = 'L', the leading N-by-N lower triangular part of B contains the lower triangular part of the matrix B. On exit, if INFO <= N, the part of B containing the matrix is overwritten by the triangular factor U or L from the Cholesky factorization B = U**T*U or B = L*L**T.
• LDB — LDB is INTEGER. The leading dimension of the array B. LDB >= max(1,N).
• W — W is REAL array, dimension (N). If INFO = 0, the eigenvalues in ascending order. slay, organized 💅
• WORK — WORK is REAL 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. If N <= 1, LWORK >= 1. If JOBZ = 'N' and N > 1, LWORK >= 2*N+1. If JOBZ = 'V' and N > 1, LWORK >= 1 + 6*N + 2*N**2. If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal sizes of the WORK and IWORK arrays, returns these values as the first entries of the WORK and IWORK arrays, and no error message related to LWORK or LIWORK is issued by XERBLA. iykyk, that's the query-mode loophole 🔥
• IWORK — IWORK is INTEGER array, dimension (MAX(1,LIWORK)). On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.
• LIWORK — LIWORK is INTEGER. The dimension of the array IWORK. If N <= 1, LIWORK >= 1. If JOBZ = 'N' and N > 1, LIWORK >= 1. If JOBZ = 'V' and N > 1, LIWORK >= 3 + 5*N. If LIWORK = -1, then a workspace query is assumed; the routine only calculates the optimal sizes of the WORK and IWORK arrays, returns these values as the first entries of the WORK and IWORK arrays, and no error message related to LWORK or LIWORK is issued by XERBLA.
• INFO — INFO is INTEGER, the exit-status bestie:
  • = 0: successful exit.
  • < 0: if INFO = -i, the i-th argument had an illegal value.
  • > 0: SPOTRF or SSYEVD returned an error code:
    • <= N: if INFO = i and JOBZ = 'N', then the algorithm failed to converge; i off-diagonal elements of an intermediate tridiagonal form did not converge to zero; if INFO = i and JOBZ = 'V', then the algorithm failed to compute an eigenvalue while working on the submatrix lying in rows and columns INFO/(N+1) through mod(INFO,N+1);
    • > N: if INFO = N + i, for 1 <= i <= N, then the leading principal minor of order i of B is not positive. The factorization of B could not be completed and no eigenvalues or eigenvectors were computed. 💀

📚 Author

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

🧚 Further Details:

Modified so that no backsubstitution is performed if SSYEVD fails to converge (NEIG in old code could be greater than N causing out of bounds reference to A - reported by Ralf Meyer). Also corrected the description of INFO and the test on ITYPE. Sven, 16 Feb 05.

💜 Contributors:

Mark Fahey, Department of Mathematics, Univ. of Kentucky, USA

Definition at line 219 of file ssygvd.f.

🌈 Author

Generated automatically by Doxygen for LAPACK from the source code.

---

and that's the whole page, bestie 🫶 this cutie is LAPACK Version 3.12.0 from source SRC/ssygvd.f(3) — no date on the footer, just so you know 👀 periodt 💖