LAPACK 3.3.1
Linear Algebra PACKage

dlabad.f

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00001       SUBROUTINE DLABAD( SMALL, LARGE )
00002 *
00003 *  -- LAPACK auxiliary routine (version 3.2) --
00004 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00005 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00006 *     November 2006
00007 *
00008 *     .. Scalar Arguments ..
00009       DOUBLE PRECISION   LARGE, SMALL
00010 *     ..
00011 *
00012 *  Purpose
00013 *  =======
00014 *
00015 *  DLABAD takes as input the values computed by DLAMCH for underflow and
00016 *  overflow, and returns the square root of each of these values if the
00017 *  log of LARGE is sufficiently large.  This subroutine is intended to
00018 *  identify machines with a large exponent range, such as the Crays, and
00019 *  redefine the underflow and overflow limits to be the square roots of
00020 *  the values computed by DLAMCH.  This subroutine is needed because
00021 *  DLAMCH does not compensate for poor arithmetic in the upper half of
00022 *  the exponent range, as is found on a Cray.
00023 *
00024 *  Arguments
00025 *  =========
00026 *
00027 *  SMALL   (input/output) DOUBLE PRECISION
00028 *          On entry, the underflow threshold as computed by DLAMCH.
00029 *          On exit, if LOG10(LARGE) is sufficiently large, the square
00030 *          root of SMALL, otherwise unchanged.
00031 *
00032 *  LARGE   (input/output) DOUBLE PRECISION
00033 *          On entry, the overflow threshold as computed by DLAMCH.
00034 *          On exit, if LOG10(LARGE) is sufficiently large, the square
00035 *          root of LARGE, otherwise unchanged.
00036 *
00037 *  =====================================================================
00038 *
00039 *     .. Intrinsic Functions ..
00040       INTRINSIC          LOG10, SQRT
00041 *     ..
00042 *     .. Executable Statements ..
00043 *
00044 *     If it looks like we're on a Cray, take the square root of
00045 *     SMALL and LARGE to avoid overflow and underflow problems.
00046 *
00047       IF( LOG10( LARGE ).GT.2000.D0 ) THEN
00048          SMALL = SQRT( SMALL )
00049          LARGE = SQRT( LARGE )
00050       END IF
00051 *
00052       RETURN
00053 *
00054 *     End of DLABAD
00055 *
00056       END
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