57 lines
940 B
C
57 lines
940 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 bessj(int n, float x)
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{
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float bessj0(float x);
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float bessj1(float x);
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void nrerror(char error_text[]);
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int j,jsum,m;
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float ax,bj,bjm,bjp,sum,tox,ans;
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if (n < 2) nrerror("Index n less than 2 in bessj");
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ax=fabs(x);
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if (ax == 0.0)
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return 0.0;
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else if (ax > (float) n) {
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tox=2.0/ax;
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bjm=bessj0(ax);
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bj=bessj1(ax);
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for (j=1;j<n;j++) {
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bjp=j*tox*bj-bjm;
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bjm=bj;
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bj=bjp;
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}
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ans=bj;
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} else {
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tox=2.0/ax;
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m=2*((n+(int) sqrt(ACC*n))/2);
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jsum=0;
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bjp=ans=sum=0.0;
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bj=1.0;
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for (j=m;j>0;j--) {
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bjm=j*tox*bj-bjp;
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bjp=bj;
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bj=bjm;
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if (fabs(bj) > BIGNO) {
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bj *= BIGNI;
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bjp *= BIGNI;
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ans *= BIGNI;
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sum *= BIGNI;
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}
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if (jsum) sum += bj;
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jsum=!jsum;
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if (j == n) ans=bjp;
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}
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sum=2.0*sum-bj;
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ans /= sum;
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}
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return x < 0.0 && (n & 1) ? -ans : ans;
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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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