47 lines
797 B
C
47 lines
797 B
C
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#include <math.h>
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#define MAXIT 100
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#define EPS 3.0e-7
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#define FPMIN 1.0e-30
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float betacf(a,b,x)
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float a,b,x;
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{
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void nrerror();
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int m,m2;
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float aa,c,d,del,h,qab,qam,qap;
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qab=a+b;
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qap=a+1.0;
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qam=a-1.0;
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c=1.0;
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d=1.0-qab*x/qap;
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if (fabs(d) < FPMIN) d=FPMIN;
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d=1.0/d;
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h=d;
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for (m=1;m<=MAXIT;m++) {
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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=1.0+aa*d;
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if (fabs(d) < FPMIN) d=FPMIN;
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c=1.0+aa/c;
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if (fabs(c) < FPMIN) c=FPMIN;
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d=1.0/d;
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h *= d*c;
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aa = -(a+m)*(qab+m)*x/((a+m2)*(qap+m2));
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d=1.0+aa*d;
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if (fabs(d) < FPMIN) d=FPMIN;
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c=1.0+aa/c;
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if (fabs(c) < FPMIN) c=FPMIN;
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d=1.0/d;
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del=d*c;
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h *= del;
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if (fabs(del-1.0) < EPS) break;
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}
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if (m > MAXIT) nrerror("a or b too big, or MAXIT too small in betacf");
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return h;
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}
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#undef MAXIT
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#undef EPS
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#undef FPMIN
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