SRC/clarrv.f(3)

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version 3.12.1 · 7 versions
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✨ SRC/clarrv.f(3) — Library Functions Manual ✨

🌸 NAME 🌸

SRC/clarrv.f — bestie, this is the page for the clarrv routine, and the synopsis line is right there: CLARRV computes the eigenvectors of the tridiagonal matrix T = L D LT given L, D and the eigenvalues of L D LT.

🦋 SYNOPSIS 🦋

🫶 Functions/Subroutines 🫶

subroutine clarrv (n, vl, vu, d, l, pivmin, isplit, m, dol, dou, minrgp,
    rtol1, rtol2, w, werr, wgap, iblock, indexw, gers, z, ldz, isuppz,
    work, iwork, info)

CLARRV computes the eigenvectors of the tridiagonal matrix T = L D LT given L, D and the eigenvalues of L D LT. Slay.

🔥 Function/Subroutine Documentation 🔥

subroutine clarrv (integer n, real vl, real vu, real, dimension( * ) d,
    real, dimension( * ) l, real pivmin, integer, dimension( * ) isplit,
    integer m, integer dol, integer dou, real minrgp, real rtol1,
    real rtol2, real, dimension( * ) w, real, dimension( * ) werr, real,
    dimension( * ) wgap, integer, dimension( * ) iblock, integer, dimension(
    * ) indexw, real, dimension( * ) gers, complex, dimension( ldz, * ) z,
    integer ldz, integer, dimension( * ) isuppz, real, dimension( * ) work,
    integer, dimension( * ) iwork, integer info)

CLARRV computes the eigenvectors of the tridiagonal matrix T = L D LT given L, D and the eigenvalues of L D LT. Iconic behavior, honestly.

🧸 Purpose 🧸

Okay so the actual purpose, straight from the source — and yes, the source wraps these lines in Doxygen !> markers, iykyk:

CLARRV computes the eigenvectors of the tridiagonal matrix T = L D L**T given L, D and APPROXIMATIONS to the eigenvalues of L D L**T. The input eigenvalues should have been computed by SLARRE.

💅 Parameters 💅

  • N — N is INTEGER. The order of the matrix. N >= 0.
  • VL — VL is REAL. Lower bound of the interval that contains the desired eigenvalues. VL < VU. Needed to compute gaps on the left or right end of the extremal eigenvalues in the desired RANGE.
  • VU — VU is REAL. Upper bound of the interval that contains the desired eigenvalues. VL < VU. Needed to compute gaps on the left or right end of the extremal eigenvalues in the desired RANGE.
  • D — D is REAL array, dimension (N). On entry, the N diagonal elements of the diagonal matrix D. On exit, D may be overwritten.
  • L — L is REAL array, dimension (N). On entry, the (N-1) subdiagonal elements of the unit bidiagonal matrix L are in elements 1 to N-1 of L (if the matrix is not split.) At the end of each block is stored the corresponding shift as given by SLARRE. On exit, L is overwritten.
  • PIVMIN — PIVMIN is REAL. The minimum pivot allowed in the Sturm sequence.
  • ISPLIT — ISPLIT is INTEGER array, dimension (N). The splitting points, at which T breaks up into blocks. The first block consists of rows/columns 1 to ISPLIT( 1 ), the second of rows/columns ISPLIT( 1 )+1 through ISPLIT( 2 ), etc.
  • M — M is INTEGER. The total number of input eigenvalues. 0 <= M <= N.
  • DOL — DOL is INTEGER. That's all the source says for this one, bestie.
  • DOU — DOU is INTEGER. If the user wants to compute only selected eigenvectors from all the eigenvalues supplied, he can specify an index range DOL:DOU. Or else the setting DOL=1, DOU=M should be applied. Note that DOL and DOU refer to the order in which the eigenvalues are stored in W. If the user wants to compute only selected eigenpairs, then the columns DOL-1 to DOU+1 of the eigenvector space Z contain the computed eigenvectors. All other columns of Z are set to zero.
  • MINRGP — MINRGP is REAL. No extra description in the source for this one.
  • RTOL1 — RTOL1 is REAL. No extra description in the source for this one.
  • RTOL2 — RTOL2 is REAL. Parameters for bisection. An interval [LEFT,RIGHT] has converged if RIGHT-LEFT < MAX( RTOL1*GAP, RTOL2*MAX(|LEFT|,|RIGHT|) ).
  • W — W is REAL array, dimension (N). The first M elements of W contain the APPROXIMATE 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 SLARRE is expected here). Furthermore, they are with respect to the shift of the corresponding root representation for their block. On exit, W holds the eigenvalues of the UNshifted matrix.
  • WERR — WERR is REAL array, dimension (N). The first M elements contain the semiwidth of the uncertainty interval of the corresponding eigenvalue in W.
  • WGAP — WGAP is REAL array, dimension (N). The separation from the right neighbor eigenvalue in W.
  • IBLOCK — IBLOCK is INTEGER array, dimension (N). The indices of the blocks (submatrices) associated with the corresponding eigenvalues in W; IBLOCK(i)=1 if eigenvalue W(i) belongs to the first block from the top, =2 if W(i) belongs to the second block, etc.
  • INDEXW — INDEXW is INTEGER array, dimension (N). The indices of the eigenvalues within each block (submatrix); for example, INDEXW(i)= 10 and IBLOCK(i)=2 imply that the i-th eigenvalue W(i) is the 10-th eigenvalue in the second block.
  • GERS — GERS is REAL array, dimension (2*N). The N Gerschgorin intervals (the i-th Gerschgorin interval is (GERS(2*i-1), GERS(2*i))). The Gerschgorin intervals should be computed from the original UNshifted matrix.
  • Z — Z is COMPLEX array, dimension (LDZ, max(1,M) ). If INFO = 0, the first M columns of Z contain the orthonormal eigenvectors of the matrix T corresponding to the input eigenvalues, with the i-th column of Z holding the eigenvector associated with W(i). Note: the user must ensure that at least max(1,M) columns are supplied in the array Z.
  • LDZ — LDZ is INTEGER. The leading dimension of the array Z. LDZ >= 1, and if JOBZ = 'V', LDZ >= max(1,N).
  • ISUPPZ — ISUPPZ is INTEGER array, dimension ( 2*max(1,M) ). The support of the eigenvectors in Z, i.e., the indices indicating the nonzero elements in Z. The I-th eigenvector is nonzero only in elements ISUPPZ( 2*I-1 ) through ISUPPZ( 2*I ).
  • WORK — WORK is REAL array, dimension (12*N).
  • IWORK — IWORK is INTEGER array, dimension (7*N).
  • INFO — INFO is INTEGER.
    • = 0: successful exit
    • > 0: A problem occurred in CLARRV.
    • < 0: One of the called subroutines signaled an internal problem. Needs inspection of the corresponding parameter IINFO for further information.
    • =-1: Problem in SLARRB when refining a child's eigenvalues.
    • =-2: Problem in SLARRF when computing the RRR of a child. When a child is inside a tight cluster, it can be difficult to find an RRR. A partial remedy from the user's point of view is to make the parameter MINRGP smaller and recompile. However, as the orthogonality of the computed vectors is proportional to 1/MINRGP, the user should be aware that he might be trading in precision when he decreases MINRGP.
    • =-3: Problem in SLARRB when refining a single eigenvalue after the Rayleigh correction was rejected.
    • = 5: The Rayleigh Quotient Iteration failed to converge to full accuracy in MAXITR steps.

👑 Author 👑

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

🎀 Contributors: 🎀

  • Beresford Parlett, University of California, Berkeley, USA
  • Jim Demmel, University of California, Berkeley, USA
  • Inderjit Dhillon, University of Texas, Austin, USA
  • Osni Marques, LBNL/NERSC, USA
  • Christof Voemel, University of California, Berkeley, USA

Definition at line 281 of file clarrv.f.

🧚 Author 🧚

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LAPACK Version 3.12.0 — SRC/clarrv.f(3). That's the source/version tea at the very end; the footer has no date, so I'm not inventing one, bestie 💜✨

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