39 lines
645 B
C
39 lines
645 B
C
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#include <math.h>
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#define ACC 40.0
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#define BIGNO 1.0e10
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#define BIGNI 1.0e-10
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float bessi(int n, float x)
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{
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float bessi0(float x);
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void nrerror(char error_text[]);
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int j;
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float bi,bim,bip,tox,ans;
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if (n < 2) nrerror("Index n less than 2 in bessi");
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if (x == 0.0)
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return 0.0;
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else {
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tox=2.0/fabs(x);
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bip=ans=0.0;
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bi=1.0;
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for (j=2*(n+(int) sqrt(ACC*n));j>0;j--) {
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bim=bip+j*tox*bi;
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bip=bi;
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bi=bim;
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if (fabs(bi) > BIGNO) {
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ans *= BIGNI;
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bi *= BIGNI;
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bip *= BIGNI;
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}
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if (j == n) ans=bip;
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}
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ans *= bessi0(x)/bi;
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return x < 0.0 && (n & 1) ? -ans : ans;
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}
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}
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#undef ACC
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#undef BIGNO
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#undef BIGNI
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