47 lines
788 B
C
47 lines
788 B
C
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#include <math.h>
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#define EULER 0.57721566
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#define MAXIT 100
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#define FPMIN 1.0e-30
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#define EPS 6.0e-8
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float ei(float x)
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{
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void nrerror(char error_text[]);
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int k;
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float fact,prev,sum,term;
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if (x <= 0.0) nrerror("Bad argument in ei");
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if (x < FPMIN) return log(x)+EULER;
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if (x <= -log(EPS)) {
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sum=0.0;
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fact=1.0;
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for (k=1;k<=MAXIT;k++) {
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fact *= x/k;
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term=fact/k;
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sum += term;
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if (term < EPS*sum) break;
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}
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if (k > MAXIT) nrerror("Series failed in ei");
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return sum+log(x)+EULER;
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} else {
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sum=0.0;
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term=1.0;
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for (k=1;k<=MAXIT;k++) {
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prev=term;
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term *= k/x;
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if (term < EPS) break;
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if (term < prev) sum += term;
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else {
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sum -= prev;
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break;
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}
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}
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return exp(x)*(1.0+sum)/x;
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}
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}
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#undef EPS
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#undef EULER
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#undef MAXIT
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#undef FPMIN
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