SRC/zstein.f(3)

💖 NAME 💖

SRC/zstein.f is the icon we’re talking about, straight out of the Library Functions Manual. Bestie, this is the ZSTEIN subroutine page, and it’s giving “linear algebra but make it understandable.”

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✨ SYNOPSIS ✨

💅 Functions/Subroutines 💅

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

ZSTEIN. That’s her. That’s the subroutine.

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🦋 Function/Subroutine Documentation 🦋

subroutine zstein (integer n, double precision, dimension( * ) d, double precision, dimension( * ) e, integer m, double precision, dimension( * ) w, integer, dimension( * ) iblock, integer, dimension( * ) isplit, complex*16, dimension( ldz, * ) z, integer ldz, double precision, dimension( * ) work, integer, dimension( * ) iwork, integer, dimension( * ) ifail, integer info)

ZSTEIN

🎀 Purpose 🎀

okay so ZSTEIN computes the eigenvectors of a real symmetric tridiagonal matrix T corresponding to specified eigenvalues, and it does this using inverse iteration. slay.

The maximum number of iterations allowed for each eigenvector is specified by an internal parameter MAXITS (currently set to 5). five. iconic. no cap.

Although the eigenvectors are real, they are stored in a complex array, which may be passed to ZUNMTR or ZUPMTR for back transformation to the eigenvectors of a complex Hermitian matrix which was reduced to tridiagonal form. iykyk.

🌈 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, stored 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 — COMPLEX*16 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. The imaginary parts of the eigenvectors are set to zero.
• 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 180 of file zstein.f.

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🌟 Author 🌟

Generated automatically by Doxygen for LAPACK from the source code.

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okay bye bestie! this page is living in LAPACK Version 3.12.0, page tag SRC/zstein.f(3) 💅✨