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SUBROUTINE STRT02( UPLO, TRANS, DIAG, N, NRHS, A, LDA, X, LDX, B,
$ LDB, WORK, RESID ) * * -- LAPACK test routine (version 3.1) -- * Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. * November 2006 * * .. Scalar Arguments .. CHARACTER DIAG, TRANS, UPLO INTEGER LDA, LDB, LDX, N, NRHS REAL RESID * .. * .. Array Arguments .. REAL A( LDA, * ), B( LDB, * ), WORK( * ), $ X( LDX, * ) * .. * * Purpose * ======= * * STRT02 computes the residual for the computed solution to a * triangular system of linear equations A*x = b or A'*x = b. * Here A is a triangular matrix, A' is the transpose of A, and x and b * are N by NRHS matrices. The test ratio is the maximum over the * number of right hand sides of * norm(b - op(A)*x) / ( norm(op(A)) * norm(x) * EPS ), * where op(A) denotes A or A' and EPS is the machine epsilon. * * Arguments * ========= * * UPLO (input) CHARACTER*1 * Specifies whether the matrix A is upper or lower triangular. * = 'U': Upper triangular * = 'L': Lower triangular * * TRANS (input) CHARACTER*1 * Specifies the operation applied to A. * = 'N': A *x = b (No transpose) * = 'T': A'*x = b (Transpose) * = 'C': A'*x = b (Conjugate transpose = Transpose) * * DIAG (input) CHARACTER*1 * Specifies whether or not the matrix A is unit triangular. * = 'N': Non-unit triangular * = 'U': Unit triangular * * N (input) INTEGER * The order of the matrix A. N >= 0. * * NRHS (input) INTEGER * The number of right hand sides, i.e., the number of columns * of the matrices X and B. NRHS >= 0. * * A (input) REAL array, dimension (LDA,N) * The triangular matrix A. If UPLO = 'U', the leading n by n * upper triangular part of the array A contains the upper * triangular matrix, and the strictly lower triangular part of * A is not referenced. If UPLO = 'L', the leading n by n lower * triangular part of the array A contains the lower triangular * matrix, and the strictly upper triangular part of A is not * referenced. If DIAG = 'U', the diagonal elements of A are * also not referenced and are assumed to be 1. * * LDA (input) INTEGER * The leading dimension of the array A. LDA >= max(1,N). * * X (input) REAL array, dimension (LDX,NRHS) * The computed solution vectors for the system of linear * equations. * * LDX (input) INTEGER * The leading dimension of the array X. LDX >= max(1,N). * * B (input) REAL array, dimension (LDB,NRHS) * The right hand side vectors for the system of linear * equations. * * LDB (input) INTEGER * The leading dimension of the array B. LDB >= max(1,N). * * WORK (workspace) REAL array, dimension (N) * * RESID (output) REAL * The maximum over the number of right hand sides of * norm(op(A)*x - b) / ( norm(op(A)) * norm(x) * EPS ). * * ===================================================================== * * .. Parameters .. REAL ZERO, ONE PARAMETER ( ZERO = 0.0E+0, ONE = 1.0E+0 ) * .. * .. Local Scalars .. INTEGER J REAL ANORM, BNORM, EPS, XNORM * .. * .. External Functions .. LOGICAL LSAME REAL SASUM, SLAMCH, SLANTR EXTERNAL LSAME, SASUM, SLAMCH, SLANTR * .. * .. External Subroutines .. EXTERNAL SAXPY, SCOPY, STRMV * .. * .. Intrinsic Functions .. INTRINSIC MAX * .. * .. Executable Statements .. * * Quick exit if N = 0 or NRHS = 0 * IF( N.LE.0 .OR. NRHS.LE.0 ) THEN RESID = ZERO RETURN END IF * * Compute the 1-norm of A or A'. * IF( LSAME( TRANS, 'N' ) ) THEN ANORM = SLANTR( '1', UPLO, DIAG, N, N, A, LDA, WORK ) ELSE ANORM = SLANTR( 'I', UPLO, DIAG, N, N, A, LDA, WORK ) END IF * * Exit with RESID = 1/EPS if ANORM = 0. * EPS = SLAMCH( 'Epsilon' ) IF( ANORM.LE.ZERO ) THEN RESID = ONE / EPS RETURN END IF * * Compute the maximum over the number of right hand sides of * norm(op(A)*x - b) / ( norm(op(A)) * norm(x) * EPS ) * RESID = ZERO DO 10 J = 1, NRHS CALL SCOPY( N, X( 1, J ), 1, WORK, 1 ) CALL STRMV( UPLO, TRANS, DIAG, N, A, LDA, WORK, 1 ) CALL SAXPY( N, -ONE, B( 1, J ), 1, WORK, 1 ) BNORM = SASUM( N, WORK, 1 ) XNORM = SASUM( N, X( 1, J ), 1 ) IF( XNORM.LE.ZERO ) THEN RESID = ONE / EPS ELSE RESID = MAX( RESID, ( ( BNORM / ANORM ) / XNORM ) / EPS ) END IF 10 CONTINUE * RETURN * * End of STRT02 * END |