?? bessj.c
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#include <math.h>#define ACC 40.0#define BIGNO 1.0e10#define BIGNI 1.0e-10float bessj(int n, float x){ float bessj0(float x); float bessj1(float x); void nrerror(char error_text[]); int j,jsum,m; float ax,bj,bjm,bjp,sum,tox,ans; if (n < 2) nrerror("Index n less than 2 in bessj"); ax=fabs(x); if (ax == 0.0) return 0.0; else if (ax > (float) n) { tox=2.0/ax; bjm=bessj0(ax); bj=bessj1(ax); for (j=1;j<n;j++) { bjp=j*tox*bj-bjm; bjm=bj; bj=bjp; } ans=bj; } else { tox=2.0/ax; m=2*((n+(int) sqrt(ACC*n))/2); jsum=0; bjp=ans=sum=0.0; bj=1.0; for (j=m;j>0;j--) { bjm=j*tox*bj-bjp; bjp=bj; bj=bjm; if (fabs(bj) > BIGNO) { bj *= BIGNI; bjp *= BIGNI; ans *= BIGNI; sum *= BIGNI; } if (jsum) sum += bj; jsum=!jsum; if (j == n) ans=bjp; } sum=2.0*sum-bj; ans /= sum; } return x < 0.0 && (n & 1) ? -ans : ans;}#undef ACC#undef BIGNO#undef BIGNI?? 快捷鍵說明
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