38 lines
919 B
Fortran
38 lines
919 B
Fortran
FUNCTION betacf(a,b,x)
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INTEGER MAXIT
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REAL betacf,a,b,x,EPS,FPMIN
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PARAMETER (MAXIT=100,EPS=3.e-7,FPMIN=1.e-30)
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INTEGER m,m2
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REAL aa,c,d,del,h,qab,qam,qap
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qab=a+b
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qap=a+1.
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qam=a-1.
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c=1.
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d=1.-qab*x/qap
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if(abs(d).lt.FPMIN)d=FPMIN
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d=1./d
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h=d
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do 11 m=1,MAXIT
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m2=2*m
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aa=m*(b-m)*x/((qam+m2)*(a+m2))
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d=1.+aa*d
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if(abs(d).lt.FPMIN)d=FPMIN
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c=1.+aa/c
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if(abs(c).lt.FPMIN)c=FPMIN
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d=1./d
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h=h*d*c
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aa=-(a+m)*(qab+m)*x/((a+m2)*(qap+m2))
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d=1.+aa*d
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if(abs(d).lt.FPMIN)d=FPMIN
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c=1.+aa/c
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if(abs(c).lt.FPMIN)c=FPMIN
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d=1./d
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del=d*c
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h=h*del
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if(abs(del-1.).lt.EPS)goto 1
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11 continue
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pause 'a or b too big, or MAXIT too small in betacf'
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1 betacf=h
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return
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END
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