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SUBROUTINE cumbet(x,y,a,b,cum,ccum)
C**********************************************************************
C
C SUBROUTINE CUMBET(X,Y,A,B,CUM,CCUM)
C Double precision cUMulative incomplete BETa distribution
C
C
C Function
C
C
C Calculates the cdf to X of the incomplete beta distribution
C with parameters a and b. This is the integral from 0 to x
C of (1/B(a,b))*f(t)) where f(t) = t**(a-1) * (1-t)**(b-1)
C
C
C Arguments
C
C
C X --> Upper limit of integration.
C X is DOUBLE PRECISION
C
C Y --> 1 - X.
C Y is DOUBLE PRECISION
C
C A --> First parameter of the beta distribution.
C A is DOUBLE PRECISION
C
C B --> Second parameter of the beta distribution.
C B is DOUBLE PRECISION
C
C CUM <-- Cumulative incomplete beta distribution.
C CUM is DOUBLE PRECISION
C
C CCUM <-- Compliment of Cumulative incomplete beta distribution.
C CCUM is DOUBLE PRECISION
C
C
C Method
C
C
C Calls the routine BRATIO.
C
C References
C
C Didonato, Armido R. and Morris, Alfred H. Jr. (1992) Algorithim
C 708 Significant Digit Computation of the Incomplete Beta Function
C Ratios. ACM ToMS, Vol.18, No. 3, Sept. 1992, 360-373.
C
C**********************************************************************
C .. Scalar Arguments ..
DOUBLE PRECISION x,y,a,b,cum,ccum
C ..
C .. Local Scalars ..
INTEGER ierr
C ..
C .. External Routines ..
EXTERNAL bratio
C ..
C .. Executable Statements ..
IF (.NOT. (x.LE.0.0D0)) GO TO 10
cum = 0.0D0
ccum = 1.0D0
RETURN
10 IF (.NOT. (y.LE.0.0D0)) GO TO 20
cum = 1.0D0
ccum = 0.0D0
RETURN
20 CALL bratio(a,b,x,y,cum,ccum,ierr)
C Call bratio routine
RETURN
END
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