LAPACK 3.12.0
LAPACK: Linear Algebra PACKage
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◆ dlarra()

subroutine dlarra ( integer n,
double precision, dimension( * ) d,
double precision, dimension( * ) e,
double precision, dimension( * ) e2,
double precision spltol,
double precision tnrm,
integer nsplit,
integer, dimension( * ) isplit,
integer info )

DLARRA computes the splitting points with the specified threshold.

Download DLARRA + dependencies [TGZ] [ZIP] [TXT]

Purpose:
!>
!> Compute the splitting points with threshold SPLTOL.
!> DLARRA sets any  off-diagonal elements to zero.
!>
Parameters
[in]N
!>          N is INTEGER
!>          The order of the matrix. N > 0.
!>
[in]D
!>          D is DOUBLE PRECISION array, dimension (N)
!>          On entry, the N diagonal elements of the tridiagonal
!>          matrix T.
!>
[in,out]E
!>          E is DOUBLE PRECISION array, dimension (N)
!>          On entry, the first (N-1) entries contain the subdiagonal
!>          elements of the tridiagonal matrix T; E(N) need not be set.
!>          On exit, the entries E( ISPLIT( I ) ), 1 <= I <= NSPLIT,
!>          are set to zero, the other entries of E are untouched.
!>
[in,out]E2
!>          E2 is DOUBLE PRECISION array, dimension (N)
!>          On entry, the first (N-1) entries contain the SQUARES of the
!>          subdiagonal elements of the tridiagonal matrix T;
!>          E2(N) need not be set.
!>          On exit, the entries E2( ISPLIT( I ) ),
!>          1 <= I <= NSPLIT, have been set to zero
!>
[in]SPLTOL
!>          SPLTOL is DOUBLE PRECISION
!>          The threshold for splitting. Two criteria can be used:
!>          SPLTOL<0 : criterion based on absolute off-diagonal value
!>          SPLTOL>0 : criterion that preserves relative accuracy
!>
[in]TNRM
!>          TNRM is DOUBLE PRECISION
!>          The norm of the matrix.
!>
[out]NSPLIT
!>          NSPLIT is INTEGER
!>          The number of blocks T splits into. 1 <= NSPLIT <= N.
!>
[out]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., and the NSPLIT-th consists of rows/columns
!>          ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N.
!>
[out]INFO
!>          INFO is INTEGER
!>          = 0:  successful exit
!>
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