dgamit.f
DOUBLE PRECISION FUNCTION DGAMIT (A, X)
C***BEGIN PROLOGUE DGAMIT
C***PURPOSE Calculate Tricomi's form of the incomplete Gamma function.
C***LIBRARY SLATEC (FNLIB)
C***CATEGORY C7E
C***TYPE DOUBLE PRECISION (GAMIT-S, DGAMIT-D)
C***KEYWORDS COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB,
C SPECIAL FUNCTIONS, TRICOMI
C***AUTHOR Fullerton, W., (LANL)
C***DESCRIPTION
C
C Evaluate Tricomi's incomplete Gamma function defined by
C
C DGAMIT = X**(-A)/GAMMA(A) * integral from 0 to X of EXP(-T) *
C T**(A-1.)
C
C for A .GT. 0.0 and by analytic continuation for A .LE. 0.0.
C GAMMA(X) is the complete gamma function of X.
C
C DGAMIT is evaluated for arbitrary real values of A and for non-
C negative values of X (even though DGAMIT is defined for X .LT.
C 0.0), except that for X = 0 and A .LE. 0.0, DGAMIT is infinite,
C which is a fatal error.
C
C The function and both arguments are DOUBLE PRECISION.
C
C A slight deterioration of 2 or 3 digits accuracy will occur when
C DGAMIT is very large or very small in absolute value, because log-
C arithmic variables are used. Also, if the parameter A is very
C close to a negative integer (but not a negative integer), there is
C a loss of accuracy, which is reported if the result is less than
C half 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, D9GMIT, D9LGIC, D9LGIT, DGAMR, 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 DGAMIT