slatec-bessel-cpp/fortran/dgamlm.f

*DECK DGAMLM
      SUBROUTINE DGAMLM (XMIN, XMAX)
C***BEGIN PROLOGUE  DGAMLM
C***PURPOSE  Compute the minimum and maximum bounds for the argument in
C            the Gamma function.
C***LIBRARY   SLATEC (FNLIB)
C***CATEGORY  C7A, R2
C***TYPE      DOUBLE PRECISION (GAMLIM-S, DGAMLM-D)
C***KEYWORDS  COMPLETE GAMMA FUNCTION, FNLIB, LIMITS, SPECIAL FUNCTIONS
C***AUTHOR  Fullerton, W., (LANL)
C***DESCRIPTION
C
C Calculate the minimum and maximum legal bounds for X in gamma(X).
C XMIN and XMAX are not the only bounds, but they are the only non-
C trivial ones to calculate.
C
C             Output Arguments --
C XMIN   double precision minimum legal value of X in gamma(X).  Any
C        smaller value of X might result in underflow.
C XMAX   double precision maximum legal value of X in gamma(X).  Any
C        larger value of X might cause overflow.
C
C***REFERENCES  (NONE)
C***ROUTINES CALLED  D1MACH, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   770601  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890531  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
C***END PROLOGUE  DGAMLM
      DOUBLE PRECISION XMIN, XMAX, ALNBIG, ALNSML, XLN, XOLD, D1MACH
C***FIRST EXECUTABLE STATEMENT  DGAMLM
      ALNSML = LOG(D1MACH(1))
      XMIN = -ALNSML
      DO 10 I=1,10
        XOLD = XMIN
        XLN = LOG(XMIN)
        XMIN = XMIN - XMIN*((XMIN+0.5D0)*XLN - XMIN - 0.2258D0 + ALNSML)
     1    / (XMIN*XLN+0.5D0)
        IF (ABS(XMIN-XOLD).LT.0.005D0) GO TO 20
 10   CONTINUE
      CALL XERMSG ('SLATEC', 'DGAMLM', 'UNABLE TO FIND XMIN', 1, 2)
C
 20   XMIN = -XMIN + 0.01D0
C
      ALNBIG = LOG (D1MACH(2))
      XMAX = ALNBIG
      DO 30 I=1,10
        XOLD = XMAX
        XLN = LOG(XMAX)
        XMAX = XMAX - XMAX*((XMAX-0.5D0)*XLN - XMAX + 0.9189D0 - ALNBIG)
     1    / (XMAX*XLN-0.5D0)
        IF (ABS(XMAX-XOLD).LT.0.005D0) GO TO 40
 30   CONTINUE
      CALL XERMSG ('SLATEC', 'DGAMLM', 'UNABLE TO FIND XMAX', 2, 2)
C
 40   XMAX = XMAX - 0.01D0
      XMIN = MAX (XMIN, -XMAX+1.D0)
C
      RETURN
      END