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44 lines
1.3 KiB
Fortran
44 lines
1.3 KiB
Fortran
*DECK DGAMR
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DOUBLE PRECISION FUNCTION DGAMR (X)
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C***BEGIN PROLOGUE DGAMR
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C***PURPOSE Compute the reciprocal of the Gamma function.
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C7A
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C***TYPE DOUBLE PRECISION (GAMR-S, DGAMR-D, CGAMR-C)
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C***KEYWORDS FNLIB, RECIPROCAL 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 DGAMR(X) calculates the double precision reciprocal of the
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C complete Gamma function for double precision argument X.
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED DGAMMA, DLGAMS, XERCLR, XGETF, XSETF
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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 900727 Added EXTERNAL statement. (WRB)
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C***END PROLOGUE DGAMR
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DOUBLE PRECISION X, ALNGX, SGNGX, DGAMMA
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EXTERNAL DGAMMA
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C***FIRST EXECUTABLE STATEMENT DGAMR
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DGAMR = 0.0D0
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IF (X.LE.0.0D0 .AND. AINT(X).EQ.X) RETURN
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C
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CALL XGETF (IROLD)
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CALL XSETF (1)
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IF (ABS(X).GT.10.0D0) GO TO 10
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DGAMR = 1.0D0/DGAMMA(X)
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CALL XERCLR
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CALL XSETF (IROLD)
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RETURN
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C
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10 CALL DLGAMS (X, ALNGX, SGNGX)
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CALL XERCLR
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CALL XSETF (IROLD)
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DGAMR = SGNGX * EXP(-ALNGX)
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RETURN
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C
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END
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