U = (float) lrand48() / RAND_MAX;
if (U == 1.0) U = 0.999999999;
R = sigma * sqrt(-2.0*log(1.0 - U));
U = (float) lrand48() / RAND_MAX;
if (U == 1.0) U = 0.999999999;
mean + R*cos(2.0*PI*U);