LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ dsvdch()

 subroutine dsvdch ( integer N, double precision, dimension( * ) S, double precision, dimension( * ) E, double precision, dimension( * ) SVD, double precision TOL, integer INFO )

DSVDCH

Purpose:
``` DSVDCH checks to see if SVD(1) ,..., SVD(N) are accurate singular
values of the bidiagonal matrix B with diagonal entries
S(1) ,..., S(N) and superdiagonal entries E(1) ,..., E(N-1)).
It does this by expanding each SVD(I) into an interval
[SVD(I) * (1-EPS) , SVD(I) * (1+EPS)], merging overlapping intervals
if any, and using Sturm sequences to count and verify whether each
resulting interval has the correct number of singular values (using
DSVDCT). Here EPS=TOL*MAX(N/10,1)*MAZHEP, where MACHEP is the
machine precision. The routine assumes the singular values are sorted
with SVD(1) the largest and SVD(N) smallest.  If each interval
contains the correct number of singular values, INFO = 0 is returned,
otherwise INFO is the index of the first singular value in the first
Parameters
 [in] N ``` N is INTEGER The dimension of the bidiagonal matrix B.``` [in] S ``` S is DOUBLE PRECISION array, dimension (N) The diagonal entries of the bidiagonal matrix B.``` [in] E ``` E is DOUBLE PRECISION array, dimension (N-1) The superdiagonal entries of the bidiagonal matrix B.``` [in] SVD ``` SVD is DOUBLE PRECISION array, dimension (N) The computed singular values to be checked.``` [in] TOL ``` TOL is DOUBLE PRECISION Error tolerance for checking, a multiplier of the machine precision.``` [out] INFO ``` INFO is INTEGER =0 if the singular values are all correct (to within 1 +- TOL*MAZHEPS) >0 if the interval containing the INFO-th singular value contains the incorrect number of singular values.```

Definition at line 96 of file dsvdch.f.

97 *
98 * -- LAPACK test routine --
99 * -- LAPACK is a software package provided by Univ. of Tennessee, --
100 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
101 *
102 * .. Scalar Arguments ..
103  INTEGER INFO, N
104  DOUBLE PRECISION TOL
105 * ..
106 * .. Array Arguments ..
107  DOUBLE PRECISION E( * ), S( * ), SVD( * )
108 * ..
109 *
110 * =====================================================================
111 *
112 * .. Parameters ..
113  DOUBLE PRECISION ONE
114  parameter( one = 1.0d0 )
115  DOUBLE PRECISION ZERO
116  parameter( zero = 0.0d0 )
117 * ..
118 * .. Local Scalars ..
119  INTEGER BPNT, COUNT, NUML, NUMU, TPNT
120  DOUBLE PRECISION EPS, LOWER, OVFL, TUPPR, UNFL, UNFLEP, UPPER
121 * ..
122 * .. External Functions ..
123  DOUBLE PRECISION DLAMCH
124  EXTERNAL dlamch
125 * ..
126 * .. External Subroutines ..
127  EXTERNAL dsvdct
128 * ..
129 * .. Intrinsic Functions ..
130  INTRINSIC max, sqrt
131 * ..
132 * .. Executable Statements ..
133 *
134 * Get machine constants
135 *
136  info = 0
137  IF( n.LE.0 )
138  \$ RETURN
139  unfl = dlamch( 'Safe minimum' )
140  ovfl = dlamch( 'Overflow' )
141  eps = dlamch( 'Epsilon' )*dlamch( 'Base' )
142 *
143 * UNFLEP is chosen so that when an eigenvalue is multiplied by the
144 * scale factor sqrt(OVFL)*sqrt(sqrt(UNFL))/MX in DSVDCT, it exceeds
145 * sqrt(UNFL), which is the lower limit for DSVDCT.
146 *
147  unflep = ( sqrt( sqrt( unfl ) ) / sqrt( ovfl ) )*svd( 1 ) +
148  \$ unfl / eps
149 *
150 * The value of EPS works best when TOL .GE. 10.
151 *
152  eps = tol*max( n / 10, 1 )*eps
153 *
154 * TPNT points to singular value at right endpoint of interval
155 * BPNT points to singular value at left endpoint of interval
156 *
157  tpnt = 1
158  bpnt = 1
159 *
160 * Begin loop over all intervals
161 *
162  10 CONTINUE
163  upper = ( one+eps )*svd( tpnt ) + unflep
164  lower = ( one-eps )*svd( bpnt ) - unflep
165  IF( lower.LE.unflep )
166  \$ lower = -upper
167 *
168 * Begin loop merging overlapping intervals
169 *
170  20 CONTINUE
171  IF( bpnt.EQ.n )
172  \$ GO TO 30
173  tuppr = ( one+eps )*svd( bpnt+1 ) + unflep
174  IF( tuppr.LT.lower )
175  \$ GO TO 30
176 *
177 * Merge
178 *
179  bpnt = bpnt + 1
180  lower = ( one-eps )*svd( bpnt ) - unflep
181  IF( lower.LE.unflep )
182  \$ lower = -upper
183  GO TO 20
184  30 CONTINUE
185 *
186 * Count singular values in interval [ LOWER, UPPER ]
187 *
188  CALL dsvdct( n, s, e, lower, numl )
189  CALL dsvdct( n, s, e, upper, numu )
190  count = numu - numl
191  IF( lower.LT.zero )
192  \$ count = count / 2
193  IF( count.NE.bpnt-tpnt+1 ) THEN
194 *
195 * Wrong number of singular values in interval
196 *
197  info = tpnt
198  GO TO 40
199  END IF
200  tpnt = bpnt + 1
201  bpnt = tpnt
202  IF( tpnt.LE.n )
203  \$ GO TO 10
204  40 CONTINUE
205  RETURN
206 *
207 * End of DSVDCH
208 *
double precision function dlamch(CMACH)
DLAMCH
Definition: dlamch.f:69
subroutine dsvdct(N, S, E, SHIFT, NUM)
DSVDCT
Definition: dsvdct.f:87
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