LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 subroutine dpot01 ( character UPLO, integer N, double precision, dimension( lda, * ) A, integer LDA, double precision, dimension( ldafac, * ) AFAC, integer LDAFAC, double precision, dimension( * ) RWORK, double precision RESID )

DPOT01

Purpose:
``` DPOT01 reconstructs a symmetric positive definite matrix  A  from
its L*L' or U'*U factorization and computes the residual
norm( L*L' - A ) / ( N * norm(A) * EPS ) or
norm( U'*U - A ) / ( N * norm(A) * EPS ),
where EPS is the machine epsilon.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 Specifies whether the upper or lower triangular part of the symmetric matrix A is stored: = 'U': Upper triangular = 'L': Lower triangular``` [in] N ``` N is INTEGER The number of rows and columns of the matrix A. N >= 0.``` [in] A ``` A is DOUBLE PRECISION array, dimension (LDA,N) The original symmetric matrix A.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N)``` [in,out] AFAC ``` AFAC is DOUBLE PRECISION array, dimension (LDAFAC,N) On entry, the factor L or U from the L*L' or U'*U factorization of A. Overwritten with the reconstructed matrix, and then with the difference L*L' - A (or U'*U - A).``` [in] LDAFAC ``` LDAFAC is INTEGER The leading dimension of the array AFAC. LDAFAC >= max(1,N).``` [out] RWORK ` RWORK is DOUBLE PRECISION array, dimension (N)` [out] RESID ``` RESID is DOUBLE PRECISION If UPLO = 'L', norm(L*L' - A) / ( N * norm(A) * EPS ) If UPLO = 'U', norm(U'*U - A) / ( N * norm(A) * EPS )```
Date
November 2011

Definition at line 106 of file dpot01.f.

106 *
107 * -- LAPACK test routine (version 3.4.0) --
108 * -- LAPACK is a software package provided by Univ. of Tennessee, --
109 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
110 * November 2011
111 *
112 * .. Scalar Arguments ..
113  CHARACTER uplo
114  INTEGER lda, ldafac, n
115  DOUBLE PRECISION resid
116 * ..
117 * .. Array Arguments ..
118  DOUBLE PRECISION a( lda, * ), afac( ldafac, * ), rwork( * )
119 * ..
120 *
121 * =====================================================================
122 *
123 * .. Parameters ..
124  DOUBLE PRECISION zero, one
125  parameter ( zero = 0.0d+0, one = 1.0d+0 )
126 * ..
127 * .. Local Scalars ..
128  INTEGER i, j, k
129  DOUBLE PRECISION anorm, eps, t
130 * ..
131 * .. External Functions ..
132  LOGICAL lsame
133  DOUBLE PRECISION ddot, dlamch, dlansy
134  EXTERNAL lsame, ddot, dlamch, dlansy
135 * ..
136 * .. External Subroutines ..
137  EXTERNAL dscal, dsyr, dtrmv
138 * ..
139 * .. Intrinsic Functions ..
140  INTRINSIC dble
141 * ..
142 * .. Executable Statements ..
143 *
144 * Quick exit if N = 0.
145 *
146  IF( n.LE.0 ) THEN
147  resid = zero
148  RETURN
149  END IF
150 *
151 * Exit with RESID = 1/EPS if ANORM = 0.
152 *
153  eps = dlamch( 'Epsilon' )
154  anorm = dlansy( '1', uplo, n, a, lda, rwork )
155  IF( anorm.LE.zero ) THEN
156  resid = one / eps
157  RETURN
158  END IF
159 *
160 * Compute the product U'*U, overwriting U.
161 *
162  IF( lsame( uplo, 'U' ) ) THEN
163  DO 10 k = n, 1, -1
164 *
165 * Compute the (K,K) element of the result.
166 *
167  t = ddot( k, afac( 1, k ), 1, afac( 1, k ), 1 )
168  afac( k, k ) = t
169 *
170 * Compute the rest of column K.
171 *
172  CALL dtrmv( 'Upper', 'Transpose', 'Non-unit', k-1, afac,
173  \$ ldafac, afac( 1, k ), 1 )
174 *
175  10 CONTINUE
176 *
177 * Compute the product L*L', overwriting L.
178 *
179  ELSE
180  DO 20 k = n, 1, -1
181 *
182 * Add a multiple of column K of the factor L to each of
183 * columns K+1 through N.
184 *
185  IF( k+1.LE.n )
186  \$ CALL dsyr( 'Lower', n-k, one, afac( k+1, k ), 1,
187  \$ afac( k+1, k+1 ), ldafac )
188 *
189 * Scale column K by the diagonal element.
190 *
191  t = afac( k, k )
192  CALL dscal( n-k+1, t, afac( k, k ), 1 )
193 *
194  20 CONTINUE
195  END IF
196 *
197 * Compute the difference L*L' - A (or U'*U - A).
198 *
199  IF( lsame( uplo, 'U' ) ) THEN
200  DO 40 j = 1, n
201  DO 30 i = 1, j
202  afac( i, j ) = afac( i, j ) - a( i, j )
203  30 CONTINUE
204  40 CONTINUE
205  ELSE
206  DO 60 j = 1, n
207  DO 50 i = j, n
208  afac( i, j ) = afac( i, j ) - a( i, j )
209  50 CONTINUE
210  60 CONTINUE
211  END IF
212 *
213 * Compute norm( L*U - A ) / ( N * norm(A) * EPS )
214 *
215  resid = dlansy( '1', uplo, n, afac, ldafac, rwork )
216 *
217  resid = ( ( resid / dble( n ) ) / anorm ) / eps
218 *
219  RETURN
220 *
221 * End of DPOT01
222 *
double precision function dlansy(NORM, UPLO, N, A, LDA, WORK)
DLANSY returns the value of the 1-norm, or the Frobenius norm, or the infinity norm, or the element of largest absolute value of a real symmetric matrix.
Definition: dlansy.f:124
double precision function dlamch(CMACH)
DLAMCH
Definition: dlamch.f:65
double precision function ddot(N, DX, INCX, DY, INCY)
DDOT
Definition: ddot.f:53
subroutine dsyr(UPLO, N, ALPHA, X, INCX, A, LDA)
DSYR
Definition: dsyr.f:134
subroutine dscal(N, DA, DX, INCX)
DSCAL
Definition: dscal.f:55
subroutine dtrmv(UPLO, TRANS, DIAG, N, A, LDA, X, INCX)
DTRMV
Definition: dtrmv.f:149
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:55

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