cpot01()

subroutine cpot01	(	character	UPLO,
		integer	N,
		complex, dimension( lda, * )	A,
		integer	LDA,
		complex, dimension( ldafac, * )	AFAC,
		integer	LDAFAC,
		real, dimension( * )	RWORK,
		real	RESID
	)

CPOT01

Purpose:

 CPOT01 reconstructs a Hermitian 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, L' is the conjugate transpose of L,
 and U' is the conjugate transpose of U.

Parameters

[in]	UPLO	UPLO is CHARACTER*1 Specifies whether the upper or lower triangular part of the Hermitian 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 COMPLEX array, dimension (LDA,N) The original Hermitian matrix A.
[in]	LDA	LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N)
[in,out]	AFAC	AFAC is COMPLEX 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 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.

Date

December 2016

Definition at line 108 of file cpot01.f.

*
*  -- LAPACK test routine (version 3.7.0) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     December 2016
*
*     .. Scalar Arguments ..
      CHARACTER          UPLO
      INTEGER            LDA, LDAFAC, N
      REAL               RESID
*     ..
*     .. Array Arguments ..
      REAL               RWORK( * )
      COMPLEX            A( LDA, * ), AFAC( LDAFAC, * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      parameter( zero = 0.0e+0, one = 1.0e+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            I, J, K
      REAL               ANORM, EPS, TR
      COMPLEX            TC
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      REAL               CLANHE, SLAMCH
      COMPLEX            CDOTC
      EXTERNAL           lsame, clanhe, slamch, cdotc
*     ..
*     .. External Subroutines ..
      EXTERNAL           cher, cscal, ctrmv
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          aimag, real
*     ..
*     .. Executable Statements ..
*
*     Quick exit if N = 0.
*
      IF( n.LE.0 ) THEN
         resid = zero
         RETURN
      END IF
*
*     Exit with RESID = 1/EPS if ANORM = 0.
*
      eps = slamch( 'Epsilon' )
      anorm = clanhe( '1', uplo, n, a, lda, rwork )
      IF( anorm.LE.zero ) THEN
         resid = one / eps
         RETURN
      END IF
*
*     Check the imaginary parts of the diagonal elements and return with
*     an error code if any are nonzero.
*
      DO 10 j = 1, n
         IF( aimag( afac( j, j ) ).NE.zero ) THEN
            resid = one / eps
            RETURN
         END IF
   10 CONTINUE
*
*     Compute the product U'*U, overwriting U.
*
      IF( lsame( uplo, 'U' ) ) THEN
         DO 20 k = n, 1, -1
*
*           Compute the (K,K) element of the result.
*
            tr = cdotc( k, afac( 1, k ), 1, afac( 1, k ), 1 )
            afac( k, k ) = tr
*
*           Compute the rest of column K.
*
            CALL ctrmv( 'Upper', 'Conjugate', 'Non-unit', k-1, afac,
     $                  ldafac, afac( 1, k ), 1 )
*
   20    CONTINUE
*
*     Compute the product L*L', overwriting L.
*
      ELSE
         DO 30 k = n, 1, -1
*
*           Add a multiple of column K of the factor L to each of
*           columns K+1 through N.
*
            IF( k+1.LE.n )
     $         CALL cher( 'Lower', n-k, one, afac( k+1, k ), 1,
     $                    afac( k+1, k+1 ), ldafac )
*
*           Scale column K by the diagonal element.
*
            tc = afac( k, k )
            CALL cscal( n-k+1, tc, afac( k, k ), 1 )
*
   30    CONTINUE
      END IF
*
*     Compute the difference  L*L' - A (or U'*U - A).
*
      IF( lsame( uplo, 'U' ) ) THEN
         DO 50 j = 1, n
            DO 40 i = 1, j - 1
               afac( i, j ) = afac( i, j ) - a( i, j )
   40       CONTINUE
            afac( j, j ) = afac( j, j ) - real( a( j, j ) )
   50    CONTINUE
      ELSE
         DO 70 j = 1, n
            afac( j, j ) = afac( j, j ) - real( a( j, j ) )
            DO 60 i = j + 1, n
               afac( i, j ) = afac( i, j ) - a( i, j )
   60       CONTINUE
   70    CONTINUE
      END IF
*
*     Compute norm( L*U - A ) / ( N * norm(A) * EPS )
*
      resid = clanhe( '1', uplo, n, afac, ldafac, rwork )
*
      resid = ( ( resid / real( n ) ) / anorm ) / eps
*
      RETURN
*
*     End of CPOT01
*

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