 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ dppt01()

 subroutine dppt01 ( character UPLO, integer N, double precision, dimension( * ) A, double precision, dimension( * ) AFAC, double precision, dimension( * ) RWORK, double precision RESID )

DPPT01

Purpose:
``` DPPT01 reconstructs a symmetric positive definite packed 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 (N*(N+1)/2) The original symmetric matrix A, stored as a packed triangular matrix.``` [in,out] AFAC ``` AFAC is DOUBLE PRECISION array, dimension (N*(N+1)/2) On entry, the factor L or U from the L*L' or U'*U factorization of A, stored as a packed triangular matrix. Overwritten with the reconstructed matrix, and then with the difference L*L' - A (or U'*U - A).``` [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 )```

Definition at line 92 of file dppt01.f.

93 *
94 * -- LAPACK test routine --
95 * -- LAPACK is a software package provided by Univ. of Tennessee, --
96 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
97 *
98 * .. Scalar Arguments ..
99  CHARACTER UPLO
100  INTEGER N
101  DOUBLE PRECISION RESID
102 * ..
103 * .. Array Arguments ..
104  DOUBLE PRECISION A( * ), AFAC( * ), RWORK( * )
105 * ..
106 *
107 * =====================================================================
108 *
109 * .. Parameters ..
110  DOUBLE PRECISION ZERO, ONE
111  parameter( zero = 0.0d+0, one = 1.0d+0 )
112 * ..
113 * .. Local Scalars ..
114  INTEGER I, K, KC, NPP
115  DOUBLE PRECISION ANORM, EPS, T
116 * ..
117 * .. External Functions ..
118  LOGICAL LSAME
119  DOUBLE PRECISION DDOT, DLAMCH, DLANSP
120  EXTERNAL lsame, ddot, dlamch, dlansp
121 * ..
122 * .. External Subroutines ..
123  EXTERNAL dscal, dspr, dtpmv
124 * ..
125 * .. Intrinsic Functions ..
126  INTRINSIC dble
127 * ..
128 * .. Executable Statements ..
129 *
130 * Quick exit if N = 0
131 *
132  IF( n.LE.0 ) THEN
133  resid = zero
134  RETURN
135  END IF
136 *
137 * Exit with RESID = 1/EPS if ANORM = 0.
138 *
139  eps = dlamch( 'Epsilon' )
140  anorm = dlansp( '1', uplo, n, a, rwork )
141  IF( anorm.LE.zero ) THEN
142  resid = one / eps
143  RETURN
144  END IF
145 *
146 * Compute the product U'*U, overwriting U.
147 *
148  IF( lsame( uplo, 'U' ) ) THEN
149  kc = ( n*( n-1 ) ) / 2 + 1
150  DO 10 k = n, 1, -1
151 *
152 * Compute the (K,K) element of the result.
153 *
154  t = ddot( k, afac( kc ), 1, afac( kc ), 1 )
155  afac( kc+k-1 ) = t
156 *
157 * Compute the rest of column K.
158 *
159  IF( k.GT.1 ) THEN
160  CALL dtpmv( 'Upper', 'Transpose', 'Non-unit', k-1, afac,
161  \$ afac( kc ), 1 )
162  kc = kc - ( k-1 )
163  END IF
164  10 CONTINUE
165 *
166 * Compute the product L*L', overwriting L.
167 *
168  ELSE
169  kc = ( n*( n+1 ) ) / 2
170  DO 20 k = n, 1, -1
171 *
172 * Add a multiple of column K of the factor L to each of
173 * columns K+1 through N.
174 *
175  IF( k.LT.n )
176  \$ CALL dspr( 'Lower', n-k, one, afac( kc+1 ), 1,
177  \$ afac( kc+n-k+1 ) )
178 *
179 * Scale column K by the diagonal element.
180 *
181  t = afac( kc )
182  CALL dscal( n-k+1, t, afac( kc ), 1 )
183 *
184  kc = kc - ( n-k+2 )
185  20 CONTINUE
186  END IF
187 *
188 * Compute the difference L*L' - A (or U'*U - A).
189 *
190  npp = n*( n+1 ) / 2
191  DO 30 i = 1, npp
192  afac( i ) = afac( i ) - a( i )
193  30 CONTINUE
194 *
195 * Compute norm( L*U - A ) / ( N * norm(A) * EPS )
196 *
197  resid = dlansp( '1', uplo, n, afac, rwork )
198 *
199  resid = ( ( resid / dble( n ) ) / anorm ) / eps
200 *
201  RETURN
202 *
203 * End of DPPT01
204 *
double precision function dlamch(CMACH)
DLAMCH
Definition: dlamch.f:69
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
double precision function ddot(N, DX, INCX, DY, INCY)
DDOT
Definition: ddot.f:82
subroutine dscal(N, DA, DX, INCX)
DSCAL
Definition: dscal.f:79
subroutine dtpmv(UPLO, TRANS, DIAG, N, AP, X, INCX)
DTPMV
Definition: dtpmv.f:142
subroutine dspr(UPLO, N, ALPHA, X, INCX, AP)
DSPR
Definition: dspr.f:127
double precision function dlansp(NORM, UPLO, N, AP, WORK)
DLANSP returns the value of the 1-norm, or the Frobenius norm, or the infinity norm,...
Definition: dlansp.f:114
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