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SUBROUTINE cumpoi(s,xlam,cum,ccum)
C**********************************************************************
C
C SUBROUTINE CUMPOI(S,XLAM,CUM,CCUM)
C CUMulative POIsson distribution
C
C
C Function
C
C
C Returns the probability of S or fewer events in a Poisson
C distribution with mean XLAM.
C
C
C Arguments
C
C
C S --> Upper limit of cumulation of the Poisson.
C S is DOUBLE PRECISION
C
C XLAM --> Mean of the Poisson distribution.
C XLAM is DOUBLE PRECIS
C
C CUM <-- Cumulative poisson distribution.
C CUM is DOUBLE PRECISION
C
C CCUM <-- Compliment of Cumulative poisson distribution.
C CCUM is DOUBLE PRECIS
C
C
C Method
C
C
C Uses formula 26.4.21 of Abramowitz and Stegun, Handbook of
C Mathematical Functions to reduce the cumulative Poisson to
C the cumulative chi-square distribution.
C
C**********************************************************************
C .. Scalar Arguments ..
DOUBLE PRECISION s,xlam,cum,ccum
C ..
C .. Local Scalars ..
DOUBLE PRECISION chi,df
C ..
C .. External Subroutines ..
EXTERNAL cumchi
C ..
C .. Executable Statements ..
df = 2.0D0* (s+1.0D0)
chi = 2.0D0*xlam
CALL cumchi(chi,df,ccum,cum)
RETURN
END
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