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*DECK DGAMI
DOUBLE PRECISION FUNCTION DGAMI (A, X)
C***BEGIN PROLOGUE DGAMI
C***PURPOSE Evaluate the incomplete Gamma function.
C***LIBRARY SLATEC (FNLIB)
C***CATEGORY C7E
C***TYPE DOUBLE PRECISION (GAMI-S, DGAMI-D)
C***KEYWORDS FNLIB, INCOMPLETE GAMMA FUNCTION, SPECIAL FUNCTIONS
C***AUTHOR Fullerton, W., (LANL)
C***DESCRIPTION
C
C Evaluate the incomplete gamma function defined by
C
C DGAMI = integral from T = 0 to X of EXP(-T) * T**(A-1.0) .
C
C DGAMI is evaluated for positive values of A and non-negative values
C of X. A slight deterioration of 2 or 3 digits accuracy will occur
C when DGAMI is very large or very small, because logarithmic variables
C are used. The function and both arguments are double precision.
C
C***REFERENCES (NONE)
C***ROUTINES CALLED DGAMIT, DLNGAM, 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***END PROLOGUE DGAMI
DOUBLE PRECISION A, X, FACTOR, DLNGAM, DGAMIT
C***FIRST EXECUTABLE STATEMENT DGAMI
IF (A .LE. 0.D0) CALL XERMSG ('SLATEC', 'DGAMI',
+ 'A MUST BE GT ZERO', 1, 2)
IF (X .LT. 0.D0) CALL XERMSG ('SLATEC', 'DGAMI',
+ 'X MUST BE GE ZERO', 2, 2)
C
DGAMI = 0.D0
IF (X.EQ.0.0D0) RETURN
C
C THE ONLY ERROR POSSIBLE IN THE EXPRESSION BELOW IS A FATAL OVERFLOW.
FACTOR = EXP (DLNGAM(A) + A*LOG(X))
C
DGAMI = FACTOR * DGAMIT (A, X)
C
RETURN
END
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