49 lines
925 B
C
49 lines
925 B
C
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#include <math.h>
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#define EPS 3.0e-14
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#define PIM4 0.7511255444649425
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#define MAXIT 10
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void gauher(float x[], float w[], int n)
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{
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void nrerror(char error_text[]);
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int i,its,j,m;
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double p1,p2,p3,pp,z,z1;
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m=(n+1)/2;
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for (i=1;i<=m;i++) {
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if (i == 1) {
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z=sqrt((double)(2*n+1))-1.85575*pow((double)(2*n+1),-0.16667);
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} else if (i == 2) {
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z -= 1.14*pow((double)n,0.426)/z;
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} else if (i == 3) {
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z=1.86*z-0.86*x[1];
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} else if (i == 4) {
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z=1.91*z-0.91*x[2];
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} else {
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z=2.0*z-x[i-2];
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}
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for (its=1;its<=MAXIT;its++) {
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p1=PIM4;
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p2=0.0;
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for (j=1;j<=n;j++) {
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p3=p2;
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p2=p1;
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p1=z*sqrt(2.0/j)*p2-sqrt(((double)(j-1))/j)*p3;
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}
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pp=sqrt((double)2*n)*p2;
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z1=z;
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z=z1-p1/pp;
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if (fabs(z-z1) <= EPS) break;
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}
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if (its > MAXIT) nrerror("too many iterations in gauher");
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x[i]=z;
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x[n+1-i] = -z;
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w[i]=2.0/(pp*pp);
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w[n+1-i]=w[i];
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}
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}
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#undef EPS
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#undef PIM4
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#undef MAXIT
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