SRC/dstein.f(3)

✨ NAME ✨

Okay so this is the page for SRC/dstein.f, chillin’ in the Library Functions Manual — and it’s all about the DSTEIN routine, bestie. 💖

🌸 SYNOPSIS 🌸

🦋 Functions/Subroutines 🦋

       subroutine dstein (n, d, e, m, w, iblock, isplit, z, ldz, work, iwork,
           ifail, info)
           DSTEIN

🦋 Function/Subroutine Documentation 🦋

Here’s the full signature, copied exactly because we do not play with Fortran:

   subroutine dstein (integer n, double precision, dimension( * ) d, double
       precision, dimension( * ) e, integer m, double precision, dimension( * )
       w, integer, dimension( * ) iblock, integer, dimension( * ) isplit,
       double precision, dimension( ldz, * ) z, integer ldz, double precision,
       dimension( * ) work, integer, dimension( * ) iwork, integer, dimension(
       * ) ifail, integer info)
       DSTEIN

💅 Purpose 💅

Okay so DSTEIN computes the eigenvectors of a real symmetric tridiagonal matrix T corresponding to specified eigenvalues, using inverse iteration. Iconic linear algebra behavior, honestly. 💜

The maximum number of iterations allowed for each eigenvector is specified by an internal parameter MAXITS (currently set to 5). Five attempts, bestie — that’s the budget. ✨

🎀 Parameters 🎀

• N — INTEGER. The order of the matrix. N >= 0.
• D — DOUBLE PRECISION array, dimension (N). The n diagonal elements of the tridiagonal matrix T.
• E — DOUBLE PRECISION array, dimension (N-1). The (n-1) subdiagonal elements of the tridiagonal matrix T, in elements 1 to N-1.
• M — INTEGER. The number of eigenvectors to be found. 0 <= M <= N.
• W — DOUBLE PRECISION array, dimension (N). The first M elements of W contain the eigenvalues for which eigenvectors are to be computed. The eigenvalues should be grouped by split-off block and ordered from smallest to largest within the block. ( The output array W from DSTEBZ with ORDER = 'B' is expected here. )
• IBLOCK — INTEGER array, dimension (N). The submatrix indices associated with the corresponding eigenvalues in W; IBLOCK(i)=1 if eigenvalue W(i) belongs to the first submatrix from the top, =2 if W(i) belongs to the second submatrix, etc. ( The output array IBLOCK from DSTEBZ is expected here. )
• ISPLIT — INTEGER array, dimension (N). The splitting points, at which T breaks up into submatrices. The first submatrix consists of rows/columns 1 to ISPLIT( 1 ), the second of rows/columns ISPLIT( 1 )+1 through ISPLIT( 2 ), etc. ( The output array ISPLIT from DSTEBZ is expected here. )
• Z — DOUBLE PRECISION array, dimension (LDZ, M). The computed eigenvectors. The eigenvector associated with the eigenvalue W(i) is stored in the i-th column of Z. Any vector which fails to converge is set to its current iterate after MAXITS iterations.
• LDZ — INTEGER. The leading dimension of the array Z. LDZ >= max(1,N).
• WORK — DOUBLE PRECISION array, dimension (5*N).
• IWORK — INTEGER array, dimension (N).
• IFAIL — INTEGER array, dimension (M). On normal exit, all elements of IFAIL are zero. If one or more eigenvectors fail to converge after MAXITS iterations, then their indices are stored in array IFAIL.
• INFO — 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 in MAXITS iterations. Their indices are stored in array IFAIL.

🧚 Internal Parameters 🧚

• MAXITS — INTEGER, default = 5. The maximum number of iterations performed.
• EXTRA — INTEGER, default = 2. The number of iterations performed after norm growth criterion is satisfied, should be at least 1.

💖 Author 💖

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

📍 Definition at line 172 of file dstein.f.

💖 Author 💖

This one’s the auto-generated bit, fr fr: Generated automatically by Doxygen for LAPACK from the source code. 🫶

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Source/version tea: this page is SRC/dstein.f(3) from LAPACK Version 3.12.0 — footer date? Not listed, so that’s all the info we’ve got. Stay convergent and iconic, bestie 💅🌈✨