SSBEVX(3)

💖 SSBEVX(3) — Library Functions Manual 💖

okay so bestie, this is the ssbevx.f(3) page from the Library Functions Manual — header path says /home/abui...RC/ssbevx.f(3) — and it’s here to break down SSBEVX like we’re in the group chat. 💅

🌸 NAME 🌸

/home/abuild/rpmbuild/BUILD/lapack-static-3.12.0-build/lapack-3.12.0/SRC/ssbevx.f — yep, that’s the file this page is about, no cap.

🦋 SYNOPSIS 🦋

✨ Functions/Subroutines ✨

       subroutine SSBEVX (jobz, range, uplo, n, kd, ab, ldab, q, ldq, vl, vu,
           il, iu, abstol, m, w, z, ldz, work, iwork, ifail, info)
            SSBEVX computes the eigenvalues and, optionally, the left and/or
           right eigenvectors for OTHER matrices

🦋 Function/Subroutine Documentation 🦋

   subroutine SSBEVX (character jobz, character range, character uplo, integer
       n, integer kd, real, dimension( ldab, * ) ab, integer ldab, real,
       dimension( ldq, * ) q, integer ldq, real vl, real vu, integer il,
       integer iu, real abstol, integer m, real, dimension( * ) w, real,
       dimension( ldz, * ) z, integer ldz, real, dimension( * ) work, integer,
       dimension( * ) iwork, integer, dimension( * ) ifail, integer info)
        SSBEVX computes the eigenvalues and, optionally, the left and/or right
       eigenvectors for OTHER matrices

✨ Purpose ✨

SSBEVX computes selected eigenvalues and, optionally, eigenvectors of a real symmetric band matrix A. Eigenvalues and eigenvectors can be selected by specifying either a range of values or a range of indices for the desired eigenvalues. iconic behavior, honestly ✨

🎀 Parameters 🎀

• JOBZ — JOBZ is CHARACTER*1.
  • = 'N': Compute eigenvalues only;
  • = 'V': Compute eigenvalues and eigenvectors.
• RANGE — RANGE is CHARACTER*1.
  • = 'A': all eigenvalues will be found;
  • = 'V': all eigenvalues in the half-open interval (VL,VU] will be found;
  • = 'I': the IL-th through IU-th eigenvalues will be found.
• UPLO — UPLO is CHARACTER*1.
  • = 'U': Upper triangle of A is stored;
  • = 'L': Lower triangle of A is stored.
• N — N is INTEGER. The order of the matrix A. N >= 0.
• KD — KD is INTEGER. The number of superdiagonals of the matrix A if UPLO = 'U', or the number of subdiagonals if UPLO = 'L'. KD >= 0.
• AB — AB is REAL array, dimension (LDAB, N). On entry, it holds the upper or lower triangle of the symmetric band matrix A, stored in the first KD+1 rows of the array. The j-th column of A is stored in the j-th column of the array AB as follows:

if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).

On exit, AB is overwritten by values generated during the reduction to tridiagonal form. If UPLO = 'U', the first superdiagonal and the diagonal of the tridiagonal matrix T are returned in rows KD and KD+1 of AB; if UPLO = 'L', the diagonal and first subdiagonal of T are returned in the first two rows of AB.

• LDAB — LDAB is INTEGER. The leading dimension of the array AB. LDAB >= KD + 1.
• Q — Q is REAL array, dimension (LDQ, N). If JOBZ = 'V', this is the N-by-N orthogonal matrix used in the reduction to tridiagonal form. If JOBZ = 'N', the array Q is not referenced.
• LDQ — LDQ is INTEGER. The leading dimension of the array Q. If JOBZ = 'V', then LDQ >= max(1,N).
• VL — VL is REAL. If RANGE='V', the lower bound of the interval to be searched for eigenvalues. VL < VU. Not referenced if RANGE = 'A' or 'I'.
• VU — VU is REAL. If RANGE='V', the upper bound of the interval to be searched for eigenvalues. VL < VU. Not referenced if RANGE = 'A' or 'I'.
• IL — IL is INTEGER. If RANGE='I', the index of the smallest eigenvalue to be returned. 1 <= IL <= IU <= N, if N > 0; IL = 1 and IU = 0 if N = 0. Not referenced if RANGE = 'A' or 'V'.
• IU — IU is INTEGER. If RANGE='I', the index of the largest eigenvalue to be returned. 1 <= IL <= IU <= N, if N > 0; IL = 1 and IU = 0 if N = 0. Not referenced if RANGE = 'A' or 'V'.
• ABSTOL — ABSTOL is REAL. The absolute error tolerance for the eigenvalues. An approximate eigenvalue is accepted as converged when it is determined to lie in an interval [a,b] of width less than or equal to

ABSTOL + EPS *   max( |a|,|b| ) ,

where EPS is the machine precision. If ABSTOL is less than or equal to zero, then EPS*|T| will be used in its place, where |T| is the 1-norm of the tridiagonal matrix obtained by reducing AB to tridiagonal form.

Eigenvalues will be computed most accurately when ABSTOL is set to twice the underflow threshold 2*SLAMCH('S'), not zero. If this routine returns with INFO>0, indicating that some eigenvectors did not converge, try setting ABSTOL to 2*SLAMCH('S').

See by Demmel and Kahan, LAPACK Working Note #3.

• M — M is INTEGER. The total number of eigenvalues found. 0 <= M <= N. If RANGE = 'A', M = N, and if RANGE = 'I', M = IU-IL+1.
• W — W is REAL array, dimension (N). The first M elements contain the selected eigenvalues in ascending order.
• Z — Z is REAL array, dimension (LDZ, max(1,M)). If JOBZ = 'V', then if INFO = 0, the first M columns of Z contain the orthonormal eigenvectors of the matrix A corresponding to the selected eigenvalues, with the i-th column of Z holding the eigenvector associated with W(i). If an eigenvector fails to converge, then that column of Z contains the latest approximation to the eigenvector, and the index of the eigenvector is returned in IFAIL. If JOBZ = 'N', then Z is not referenced.

Note: the user must ensure that at least max(1,M) columns are supplied in the array Z; if RANGE = 'V', the exact value of M is not known in advance and an upper bound must be used. this is the “bring enough snacks” of array sizing, bestie 🍓

• LDZ — LDZ is INTEGER. The leading dimension of the array Z. LDZ >= 1, and if JOBZ = 'V', LDZ >= max(1,N).
• WORK — WORK is REAL array, dimension (7*N).
• IWORK — IWORK is INTEGER array, dimension (5*N).
• IFAIL — IFAIL is INTEGER array, dimension (N). If JOBZ = 'V', then if INFO = 0, the first M elements of IFAIL are zero. If INFO > 0, then IFAIL contains the indices of the eigenvectors that failed to converge. If JOBZ = 'N', then IFAIL is not referenced.
• INFO — INFO is INTEGER.

= 0:  successful exit.
< 0:  if INFO = -i, the i-th argument had an illegal value.
> 0:  if INFO = i, then i eigenvectors failed to converge.
      Their indices are stored in array IFAIL.

💜 Author 💜

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

📌 Definition at line 262 of file ssbevx.f. 📌

Definition at line 262 of file ssbevx.f.

👑 Author 👑

Generated automatically by Doxygen for LAPACK from the source code.

---

Mwah 💋 — this page is LAPACK Version 3.12.0 for /home/abui...RC/ssbevx.f(3), and it was generated automatically by Doxygen for LAPACK from the source code. Stay sparkling, bestie. 🫶✨