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73 lines
2.2 KiB
Fortran
73 lines
2.2 KiB
Fortran
*DECK BINOM
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FUNCTION BINOM (N, M)
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C***BEGIN PROLOGUE BINOM
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C***PURPOSE Compute the binomial coefficients.
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C1
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C***TYPE SINGLE PRECISION (BINOM-S, DBINOM-D)
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C***KEYWORDS BINOMIAL COEFFICIENTS, FNLIB, 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 BINOM(N,M) calculates the binomial coefficient (N!)/((M!)*(N-M)!).
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED ALNREL, R1MACH, R9LGMC, 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 900326 Removed duplicate information from DESCRIPTION section.
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C (WRB)
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C***END PROLOGUE BINOM
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LOGICAL FIRST
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SAVE SQ2PIL, BILNMX, FINTMX, FIRST
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DATA SQ2PIL / 0.9189385332 0467274E0 /
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DATA FIRST /.TRUE./
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C***FIRST EXECUTABLE STATEMENT BINOM
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IF (FIRST) THEN
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BILNMX = LOG (R1MACH(2))
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FINTMX = 0.9/R1MACH(3)
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ENDIF
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FIRST = .FALSE.
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C
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IF (N .LT. 0 .OR. M .LT. 0) CALL XERMSG ('SLATEC', 'BINOM',
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+ 'N OR M LT ZERO', 1, 2)
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IF (N .LT. M) CALL XERMSG ('SLATEC', 'BINOM', 'N LT M', 2, 2)
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C
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K = MIN (M, N-M)
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IF (K.GT.20) GO TO 30
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IF (K*LOG(AMAX0(N,1)).GT.BILNMX) GO TO 30
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C
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BINOM = 1.
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IF (K.EQ.0) RETURN
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C
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DO 20 I=1,K
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BINOM = BINOM * REAL(N-I+1)/I
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20 CONTINUE
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C
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IF (BINOM.LT.FINTMX) BINOM = AINT (BINOM+0.5)
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RETURN
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C
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C IF K.LT.9, APPROX IS NOT VALID AND ANSWER IS CLOSE TO THE OVERFLOW LIM
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30 IF (K .LT. 9) CALL XERMSG ('SLATEC', 'BINOM',
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+ 'RESULT OVERFLOWS BECAUSE N AND/OR M TOO BIG', 3, 2)
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C
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XN = N + 1
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XK = K + 1
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XNK = N - K + 1
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C
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CORR = R9LGMC(XN) - R9LGMC(XK) - R9LGMC(XNK)
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BINOM = XK*LOG(XNK/XK) - XN*ALNREL(-(XK-1.)/XN)
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1 - 0.5*LOG(XN*XNK/XK) + 1.0 - SQ2PIL + CORR
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C
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IF (BINOM .GT. BILNMX) CALL XERMSG ('SLATEC', 'BINOM',
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+ 'RESULT OVERFLOWS BECAUSE N AND/OR M TOO BIG', 3, 2)
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C
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BINOM = EXP (BINOM)
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IF (BINOM.LT.FINTMX) BINOM = AINT (BINOM+0.5)
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C
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RETURN
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END
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