LAPACK  3.8.0
LAPACK: Linear Algebra PACKage

◆ 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 "small" 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.
Date
June 2017
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 138 of file dlarra.f.

138 *
139 * -- LAPACK auxiliary routine (version 3.7.1) --
140 * -- LAPACK is a software package provided by Univ. of Tennessee, --
141 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
142 * June 2017
143 *
144 * .. Scalar Arguments ..
145  INTEGER info, n, nsplit
146  DOUBLE PRECISION spltol, tnrm
147 * ..
148 * .. Array Arguments ..
149  INTEGER isplit( * )
150  DOUBLE PRECISION d( * ), e( * ), e2( * )
151 * ..
152 *
153 * =====================================================================
154 *
155 * .. Parameters ..
156  DOUBLE PRECISION zero
157  parameter( zero = 0.0d0 )
158 * ..
159 * .. Local Scalars ..
160  INTEGER i
161  DOUBLE PRECISION eabs, tmp1
162 
163 * ..
164 * .. Intrinsic Functions ..
165  INTRINSIC abs
166 * ..
167 * .. Executable Statements ..
168 *
169  info = 0
170 *
171 * Quick return if possible
172 *
173  IF( n.LE.0 ) THEN
174  RETURN
175  END IF
176 *
177 * Compute splitting points
178  nsplit = 1
179  IF(spltol.LT.zero) THEN
180 * Criterion based on absolute off-diagonal value
181  tmp1 = abs(spltol)* tnrm
182  DO 9 i = 1, n-1
183  eabs = abs( e(i) )
184  IF( eabs .LE. tmp1) THEN
185  e(i) = zero
186  e2(i) = zero
187  isplit( nsplit ) = i
188  nsplit = nsplit + 1
189  END IF
190  9 CONTINUE
191  ELSE
192 * Criterion that guarantees relative accuracy
193  DO 10 i = 1, n-1
194  eabs = abs( e(i) )
195  IF( eabs .LE. spltol * sqrt(abs(d(i)))*sqrt(abs(d(i+1))) )
196  $ THEN
197  e(i) = zero
198  e2(i) = zero
199  isplit( nsplit ) = i
200  nsplit = nsplit + 1
201  END IF
202  10 CONTINUE
203  ENDIF
204  isplit( nsplit ) = n
205 
206  RETURN
207 *
208 * End of DLARRA
209 *
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