probks.c

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#include <math.h>
#define EPS1 0.001
#define EPS2 1.0e-8

float probks(float alam)
{
	int j;
	float a2,fac=2.0,sum=0.0,term,termbf=0.0;

	a2 = -2.0*alam*alam;
	for (j=1;j<=100;j++) {
		term=fac*exp(a2*j*j);
		sum += term;
		if (fabs(term) <= EPS1*termbf || fabs(term) <= EPS2*sum) return sum;
		fac = -fac;
		termbf=fabs(term);
	}
	return 1.0;
}
#undef EPS1
#undef EPS2