SPTEQR(3)

💖 SPTEQR — spteqr.f(3) 💖

okay bestie, welcome to the Library Functions Manual page for spteqr.f(3). The header path is the truncated /home/abui...RC/spteqr.f(3), and the full source path is in NAME below. This is LAPACK doing tridiagonal eigenvalue/eigenvector things with high relative accuracy, and it is lowkey iconic. no cap 💅

🌸 NAME 🌸

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

That path is the whole NAME entry, periodt.

✨ SYNOPSIS ✨

Functions/Subroutines

subroutine SPTEQR (compz, n, d, e, z, ldz, work, info)

SPTEQR

🫶 Function/Subroutine Documentation 🫶

subroutine SPTEQR

   subroutine SPTEQR (character compz, integer n, real, dimension( * ) d, real,
       dimension( * ) e, real, dimension( ldz, * ) z, integer ldz, real,
       dimension( * ) work, integer info)

SPTEQR

💜 Purpose 💜

SPTEQR computes all eigenvalues and, optionally, eigenvectors of a symmetric positive definite tridiagonal matrix. The way it does this: first it factors the matrix using SPTTRF, and then it calls SBDSQR to compute the singular values of the bidiagonal factor. slay.

This routine computes the eigenvalues of the positive definite tridiagonal matrix to high relative accuracy. That means if the eigenvalues range over many orders of magnitude in size, then the small eigenvalues and corresponding eigenvectors will be computed more accurately than, for example, with the standard QR method. we love precision 💖

The eigenvectors of a full or band symmetric positive definite matrix can also be found if SSYTRD, SSPTRD, or SSBTRD has been used to reduce this matrix to tridiagonal form. heads up though: the reduction to tridiagonal form may preclude the possibility of obtaining high relative accuracy in the small eigenvalues of the original matrix, if these eigenvalues range over many orders of magnitude. bestie, that caveat is real.

🎀 Parameters 🎀

COMPZ

COMPZ is CHARACTER*1

• = 'N': Compute eigenvalues only.
• = 'V': Compute eigenvectors of original symmetric matrix also. Array Z contains the orthogonal matrix used to reduce the original matrix to tridiagonal form.
• = 'I': Compute eigenvectors of tridiagonal matrix also.

N

N is INTEGER

The order of the matrix. N >= 0.

D

D is REAL array, dimension (N)

On entry, the n diagonal elements of the tridiagonal matrix.

On normal exit, D contains the eigenvalues, in descending order.

E

E is REAL array, dimension (N-1)

On entry, the (n-1) subdiagonal elements of the tridiagonal matrix.

On exit, E has been destroyed. 💀

Z

Z is REAL array, dimension (LDZ, N)

On entry, if COMPZ = 'V', the orthogonal matrix used in the reduction to tridiagonal form.

On exit, if COMPZ = 'V', the orthonormal eigenvectors of the original symmetric matrix; if COMPZ = 'I', the orthonormal eigenvectors of the tridiagonal matrix.

If INFO > 0 on exit, Z contains the eigenvectors associated with only the stored eigenvalues.

If COMPZ = 'N', then Z is not referenced.

LDZ

LDZ is INTEGER

The leading dimension of the array Z. LDZ >= 1, and if COMPZ = 'V' or 'I', LDZ >= max(1,N).

WORK

WORK is REAL array, dimension (4*N)

INFO

INFO is INTEGER

• = 0: successful exit.
• < 0: if INFO = -i, the i-th argument had an illegal value.
• > 0: if INFO = i, and i is:
  • <= N: the Cholesky factorization of the matrix could not be performed because the leading principal minor of order i was not positive.
  • > N: the SVD algorithm failed to converge; if INFO = N+i, i off-diagonal elements of the bidiagonal factor did not converge to zero.

👑 Author 👑

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

Definition at line 144 of file spteqr.f.

🤖 Author 🤖

Generated automatically by Doxygen for LAPACK from the source code.

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xoxo, source/version is LAPACK Version 3.12.0, footer path is /home/abui...RC/spteqr.f(3), and no date is listed on this footer, bestie 🫶