dgamic.f
DOUBLE PRECISION FUNCTION DGAMIC (A, X)
C***BEGIN PROLOGUE DGAMIC
C***PURPOSE Calculate the complementary incomplete Gamma function.
C***LIBRARY SLATEC (FNLIB)
C***CATEGORY C7E
C***TYPE DOUBLE PRECISION (GAMIC-S, DGAMIC-D)
C***KEYWORDS COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB,
C SPECIAL FUNCTIONS
C***AUTHOR Fullerton, W., (LANL)
C***DESCRIPTION
C
C Evaluate the complementary incomplete Gamma function
C
C DGAMIC = integral from X to infinity of EXP(-T) * T**(A-1.) .
C
C DGAMIC is evaluated for arbitrary real values of A and for non-
C negative values of X (even though DGAMIC is defined for X .LT.
C 0.0), except that for X = 0 and A .LE. 0.0, DGAMIC is undefined.
C
C DGAMIC, A, and X are DOUBLE PRECISION.
C
C A slight deterioration of 2 or 3 digits accuracy will occur when
C DGAMIC is very large or very small in absolute value, because log-
C arithmic variables are used. Also, if the parameter A is very close
C to a negative INTEGER (but not a negative integer), there is a loss
C of accuracy, which is reported if the result is less than half
C machine precision.
C
C***REFERENCES W. Gautschi, A computational procedure for incomplete
C gamma functions, ACM Transactions on Mathematical
C Software 5, 4 (December 1979), pp. 466-481.
C W. Gautschi, Incomplete gamma functions, Algorithm 542,
C ACM Transactions on Mathematical Software 5, 4
C (December 1979), pp. 482-489.
C***ROUTINES CALLED D1MACH, D9GMIC, D9GMIT, D9LGIC, D9LGIT, DLGAMS,
C DLNGAM, XERCLR, XERMSG
C***REVISION HISTORY (YYMMDD)
C 770701 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 920528 DESCRIPTION and REFERENCES sections revised. (WRB)
C***END PROLOGUE DGAMIC