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46 lines
1.6 KiB
Fortran
46 lines
1.6 KiB
Fortran
*DECK DGAMI
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DOUBLE PRECISION FUNCTION DGAMI (A, X)
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C***BEGIN PROLOGUE DGAMI
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C***PURPOSE Evaluate the incomplete Gamma function.
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C7E
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C***TYPE DOUBLE PRECISION (GAMI-S, DGAMI-D)
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C***KEYWORDS FNLIB, INCOMPLETE GAMMA FUNCTION, SPECIAL FUNCTIONS
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C***AUTHOR Fullerton, W., (LANL)
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C***DESCRIPTION
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C
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C Evaluate the incomplete gamma function defined by
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C
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C DGAMI = integral from T = 0 to X of EXP(-T) * T**(A-1.0) .
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C
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C DGAMI is evaluated for positive values of A and non-negative values
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C of X. A slight deterioration of 2 or 3 digits accuracy will occur
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C when DGAMI is very large or very small, because logarithmic variables
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C are used. The function and both arguments are double precision.
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED DGAMIT, DLNGAM, XERMSG
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C***REVISION HISTORY (YYMMDD)
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C 770701 DATE WRITTEN
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C 890531 Changed all specific intrinsics to generic. (WRB)
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C 890531 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ)
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C***END PROLOGUE DGAMI
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DOUBLE PRECISION A, X, FACTOR, DLNGAM, DGAMIT
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C***FIRST EXECUTABLE STATEMENT DGAMI
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IF (A .LE. 0.D0) CALL XERMSG ('SLATEC', 'DGAMI',
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+ 'A MUST BE GT ZERO', 1, 2)
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IF (X .LT. 0.D0) CALL XERMSG ('SLATEC', 'DGAMI',
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+ 'X MUST BE GE ZERO', 2, 2)
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C
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DGAMI = 0.D0
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IF (X.EQ.0.0D0) RETURN
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C
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C THE ONLY ERROR POSSIBLE IN THE EXPRESSION BELOW IS A FATAL OVERFLOW.
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FACTOR = EXP (DLNGAM(A) + A*LOG(X))
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C
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DGAMI = FACTOR * DGAMIT (A, X)
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C
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RETURN
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END
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