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73 lines
2.4 KiB
Fortran
73 lines
2.4 KiB
Fortran
*DECK DLNGAM
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DOUBLE PRECISION FUNCTION DLNGAM (X)
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C***BEGIN PROLOGUE DLNGAM
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C***PURPOSE Compute the logarithm of the absolute value of the Gamma
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C function.
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C7A
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C***TYPE DOUBLE PRECISION (ALNGAM-S, DLNGAM-D, CLNGAM-C)
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C***KEYWORDS ABSOLUTE VALUE, COMPLETE GAMMA FUNCTION, FNLIB, LOGARITHM,
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C 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 DLNGAM(X) calculates the double precision logarithm of the
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C absolute value of the Gamma function for double precision
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C argument X.
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED D1MACH, D9LGMC, DGAMMA, XERMSG
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C***REVISION HISTORY (YYMMDD)
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C 770601 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 900727 Added EXTERNAL statement. (WRB)
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C***END PROLOGUE DLNGAM
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DOUBLE PRECISION X, DXREL, PI, SINPIY, SQPI2L, SQ2PIL, XMAX,
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1 Y, DGAMMA, D9LGMC, D1MACH, TEMP
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LOGICAL FIRST
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EXTERNAL DGAMMA
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SAVE SQ2PIL, SQPI2L, PI, XMAX, DXREL, FIRST
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DATA SQ2PIL / 0.9189385332 0467274178 0329736405 62 D0 /
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DATA SQPI2L / +.2257913526 4472743236 3097614947 441 D+0 /
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DATA PI / 3.1415926535 8979323846 2643383279 50 D0 /
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DATA FIRST /.TRUE./
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C***FIRST EXECUTABLE STATEMENT DLNGAM
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IF (FIRST) THEN
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TEMP = 1.D0/LOG(D1MACH(2))
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XMAX = TEMP*D1MACH(2)
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DXREL = SQRT(D1MACH(4))
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ENDIF
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FIRST = .FALSE.
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C
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Y = ABS (X)
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IF (Y.GT.10.D0) GO TO 20
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C
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C LOG (ABS (DGAMMA(X)) ) FOR ABS(X) .LE. 10.0
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C
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DLNGAM = LOG (ABS (DGAMMA(X)) )
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RETURN
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C
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C LOG ( ABS (DGAMMA(X)) ) FOR ABS(X) .GT. 10.0
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C
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20 IF (Y .GT. XMAX) CALL XERMSG ('SLATEC', 'DLNGAM',
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+ 'ABS(X) SO BIG DLNGAM OVERFLOWS', 2, 2)
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C
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IF (X.GT.0.D0) DLNGAM = SQ2PIL + (X-0.5D0)*LOG(X) - X + D9LGMC(Y)
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IF (X.GT.0.D0) RETURN
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C
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SINPIY = ABS (SIN(PI*Y))
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IF (SINPIY .EQ. 0.D0) CALL XERMSG ('SLATEC', 'DLNGAM',
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+ 'X IS A NEGATIVE INTEGER', 3, 2)
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C
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IF (ABS((X-AINT(X-0.5D0))/X) .LT. DXREL) CALL XERMSG ('SLATEC',
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+ 'DLNGAM',
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+ 'ANSWER LT HALF PRECISION BECAUSE X TOO NEAR NEGATIVE INTEGER',
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+ 1, 1)
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C
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DLNGAM = SQPI2L + (X-0.5D0)*LOG(Y) - X - LOG(SINPIY) - D9LGMC(Y)
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RETURN
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C
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END
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