 
      DOUBLE PRECISION FUNCTION DGAMIT(A,X)
C***BEGIN PROLOGUE  DGAMIT
C***DATE WRITTEN   770701   (YYMMDD)
C***REVISION DATE  820801   (YYMMDD)
C***CATEGORY NO.  C7E
C***KEYWORDS  COMPLEMENTARY,COMPLEMENTARY INCOMPLETE GAMMA FUNCTION,
C             DOUBLE PRECISION,GAMMA FUNCTION,SPECIAL FUNCTION,TRICOMI
C***AUTHOR  FULLERTON, W., (LANL)
C***PURPOSE  Calculates Tricomi's form of the incomplete Gamma function.
C***DESCRIPTION
C
C Evaluate Tricomi's incomplete gamma function defined by
C
C DGAMIT = X**(-A)/GAMMA(A) * integral T = 0 to X of EXP(-T) * T**(A-1.)
C
C for A .GT. 0.0 and by analytic
C continuation for A .LE. 0.0.  Gamma(X) is the complete
C gamma function of X.  DGAMIT is evaluated for arbitrary real values of
C A and for non-negative values of X (even though DGAMIT is defined for
C X .LT. 0.0), except that for X = 0 and A .LE. 0.0, DGAMIT is infinite,
C a fatal error.  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 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 Ref. -- W. Gautschi, An Evaluation Procedure for Incomplete Gamma
C Functions, ACM Trans. Math. Software, Vol. 5, No. 4, December 1979.
C***REFERENCES  (NONE)
C***ROUTINES CALLED  D1MACH,D9GMIT,D9LGIC,D9LGIT,DGAMR,DINT,DLGAMS,
C                    DLNGAM,XERCLR,XERROR
C***END PROLOGUE  DGAMIT
 
 
