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53 lines
1.5 KiB
Fortran
53 lines
1.5 KiB
Fortran
*DECK R9LGIC
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FUNCTION R9LGIC (A, X, ALX)
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C***BEGIN PROLOGUE R9LGIC
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C***SUBSIDIARY
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C***PURPOSE Compute the log complementary incomplete Gamma function
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C for large X and for A .LE. X.
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C7E
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C***TYPE SINGLE PRECISION (R9LGIC-S, D9LGIC-D)
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C***KEYWORDS COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB, LARGE X,
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C LOGARITHM, 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 Compute the log complementary incomplete gamma function for large X
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C and for A .LE. X.
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED R1MACH, 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 900720 Routine changed from user-callable to subsidiary. (WRB)
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C***END PROLOGUE R9LGIC
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SAVE EPS
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DATA EPS / 0.0 /
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C***FIRST EXECUTABLE STATEMENT R9LGIC
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IF (EPS.EQ.0.0) EPS = 0.5*R1MACH(3)
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C
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XPA = X + 1.0 - A
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XMA = X - 1.0 - A
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C
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R = 0.0
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P = 1.0
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S = P
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DO 10 K=1,200
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FK = K
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T = FK*(A-FK)*(1.0+R)
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R = -T/((XMA+2.0*FK)*(XPA+2.0*FK)+T)
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P = R*P
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S = S + P
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IF (ABS(P).LT.EPS*S) GO TO 20
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10 CONTINUE
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CALL XERMSG ('SLATEC', 'R9LGIC',
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+ 'NO CONVERGENCE IN 200 TERMS OF CONTINUED FRACTION', 1, 2)
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C
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20 R9LGIC = A*ALX - X + LOG(S/XPA)
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C
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RETURN
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END
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