dlarra (l)  Linux Manuals
dlarra: the splitting points with threshold SPLTOL
NAME
DLARRA  the splitting points with threshold SPLTOLSYNOPSIS
 SUBROUTINE DLARRA(
 N, D, E, E2, SPLTOL, TNRM, NSPLIT, ISPLIT, INFO )
 IMPLICIT NONE
 INTEGER INFO, N, NSPLIT
 DOUBLE PRECISION SPLTOL, TNRM
 INTEGER ISPLIT( * )
 DOUBLE PRECISION D( * ), E( * ), E2( * )
PURPOSE
Compute the splitting points with threshold SPLTOL. DLARRA sets any "small" offdiagonal elements to zero.ARGUMENTS
 N (input) INTEGER
 The order of the matrix. N > 0.
 D (input) DOUBLE PRECISION array, dimension (N)
 On entry, the N diagonal elements of the tridiagonal matrix T.
 E (input/output) DOUBLE PRECISION array, dimension (N)
 On entry, the first (N1) 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.
 E2 (input/output) DOUBLE PRECISION array, dimension (N)

On entry, the first (N1) 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
SPLTOL (input) DOUBLE PRECISION
The threshold for splitting. Two criteria can be used:
SPLTOL<0 : criterion based on absolute offdiagonal value
SPLTOL>0 : criterion that preserves relative accuracy TNRM (input) DOUBLE PRECISION The norm of the matrix.  NSPLIT (output) INTEGER
 The number of blocks T splits into. 1 <= NSPLIT <= N.
 ISPLIT (output) 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 NSPLITth consists of rows/columns ISPLIT(NSPLIT1)+1 through ISPLIT(NSPLIT)=N.
 INFO (output) INTEGER
 = 0: successful exit
FURTHER DETAILS
Based on contributions byBeresford 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