SLATEC Routines --- DGAMIT ---

*DECK DGAMIT
      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