LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ sppt01()

subroutine sppt01 ( character  UPLO,
integer  N,
real, dimension( * )  A,
real, dimension( * )  AFAC,
real, dimension( * )  RWORK,
real  RESID 
)

SPPT01

Purpose:
 SPPT01 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 REAL array, dimension (N*(N+1)/2)
          The original symmetric matrix A, stored as a packed
          triangular matrix.
[in,out]AFAC
          AFAC is REAL 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 REAL array, dimension (N)
[out]RESID
          RESID is REAL
          If UPLO = 'L', norm(L*L' - A) / ( N * norm(A) * EPS )
          If UPLO = 'U', norm(U'*U - A) / ( N * norm(A) * EPS )
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 92 of file sppt01.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  REAL RESID
102 * ..
103 * .. Array Arguments ..
104  REAL A( * ), AFAC( * ), RWORK( * )
105 * ..
106 *
107 * =====================================================================
108 *
109 * .. Parameters ..
110  REAL ZERO, ONE
111  parameter( zero = 0.0e+0, one = 1.0e+0 )
112 * ..
113 * .. Local Scalars ..
114  INTEGER I, K, KC, NPP
115  REAL ANORM, EPS, T
116 * ..
117 * .. External Functions ..
118  LOGICAL LSAME
119  REAL SDOT, SLAMCH, SLANSP
120  EXTERNAL lsame, sdot, slamch, slansp
121 * ..
122 * .. External Subroutines ..
123  EXTERNAL sscal, sspr, stpmv
124 * ..
125 * .. Intrinsic Functions ..
126  INTRINSIC real
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 = slamch( 'Epsilon' )
140  anorm = slansp( '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 = sdot( 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 stpmv( '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 sspr( '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 sscal( 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 = slansp( '1', uplo, n, afac, rwork )
198 *
199  resid = ( ( resid / real( n ) ) / anorm ) / eps
200 *
201  RETURN
202 *
203 * End of SPPT01
204 *
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
real function slansp(NORM, UPLO, N, AP, WORK)
SLANSP returns the value of the 1-norm, or the Frobenius norm, or the infinity norm,...
Definition: slansp.f:114
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79
real function sdot(N, SX, INCX, SY, INCY)
SDOT
Definition: sdot.f:82
subroutine sspr(UPLO, N, ALPHA, X, INCX, AP)
SSPR
Definition: sspr.f:127
subroutine stpmv(UPLO, TRANS, DIAG, N, AP, X, INCX)
STPMV
Definition: stpmv.f:142
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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