strt06.f

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      SUBROUTINE STRT06( RCOND, RCONDC, UPLO, DIAG, N, A, LDA, WORK,
     $                   RAT )
*
*  -- LAPACK test routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          DIAG, UPLO
      INTEGER            LDA, N
      REAL               RAT, RCOND, RCONDC
*     ..
*     .. Array Arguments ..
      REAL               A( LDA, * ), WORK( * )
*     ..
*
*  Purpose
*  =======
*
*  STRT06 computes a test ratio comparing RCOND (the reciprocal
*  condition number of a triangular matrix A) and RCONDC, the estimate
*  computed by STRCON.  Information about the triangular matrix A is
*  used if one estimate is zero and the other is non-zero to decide if
*  underflow in the estimate is justified.
*
*  Arguments
*  =========
*
*  RCOND   (input) REAL
*          The estimate of the reciprocal condition number obtained by
*          forming the explicit inverse of the matrix A and computing
*          RCOND = 1/( norm(A) * norm(inv(A)) ).
*
*  RCONDC  (input) REAL
*          The estimate of the reciprocal condition number computed by
*          STRCON.
*
*  UPLO    (input) CHARACTER
*          Specifies whether the matrix A is upper or lower triangular.
*          = 'U':  Upper triangular
*          = 'L':  Lower triangular
*
*  DIAG    (input) CHARACTER
*          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.
*
*  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).
*
*  WORK    (workspace) REAL array, dimension (N)
*
*  RAT     (output) REAL
*          The test ratio.  If both RCOND and RCONDC are nonzero,
*             RAT = MAX( RCOND, RCONDC )/MIN( RCOND, RCONDC ) - 1.
*          If RAT = 0, the two estimates are exactly the same.
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ZERO, ONE
      PARAMETER          ( ZERO = 0.0E+0, ONE = 1.0E+0 )
*     ..
*     .. Local Scalars ..
      REAL               ANORM, BIGNUM, EPS, RMAX, RMIN, SMLNUM
*     ..
*     .. External Functions ..
      REAL               SLAMCH, SLANTR
      EXTERNAL           SLAMCH, SLANTR
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX, MIN
*     ..
*     .. External Subroutines ..
      EXTERNAL           SLABAD
*     ..
*     .. Executable Statements ..
*
      EPS = SLAMCH( 'Epsilon' )
      RMAX = MAX( RCOND, RCONDC )
      RMIN = MIN( RCOND, RCONDC )
*
*     Do the easy cases first.
*
      IF( RMIN.LT.ZERO ) THEN
*
*        Invalid value for RCOND or RCONDC, return 1/EPS.
*
         RAT = ONE / EPS
*
      ELSE IF( RMIN.GT.ZERO ) THEN
*
*        Both estimates are positive, return RMAX/RMIN - 1.
*
         RAT = RMAX / RMIN - ONE
*
      ELSE IF( RMAX.EQ.ZERO ) THEN
*
*        Both estimates zero.
*
         RAT = ZERO
*
      ELSE
*
*        One estimate is zero, the other is non-zero.  If the matrix is
*        ill-conditioned, return the nonzero estimate multiplied by
*        1/EPS; if the matrix is badly scaled, return the nonzero
*        estimate multiplied by BIGNUM/TMAX, where TMAX is the maximum
*        element in absolute value in A.
*
         SMLNUM = SLAMCH( 'Safe minimum' )
         BIGNUM = ONE / SMLNUM
         CALL SLABAD( SMLNUM, BIGNUM )
         ANORM = SLANTR( 'M', UPLO, DIAG, N, N, A, LDA, WORK )
*
         RAT = RMAX*( MIN( BIGNUM / MAX( ONE, ANORM ), ONE / EPS ) )
      END IF
*
      RETURN
*
*     End of STRT06
*
      END