//   evaluate the integral with importance sampling
     for ( int i = 1;  i <= n; i++){
       x = ran0(&idum);  // random numbers in [0,1]
       y = 2 - sqrt(4-3*x);  // new random numbers
       fy=3*func(y)/(4-2*y); // weighted function
       int_mc += fy;
       sum_sigma += fy*fy;
     }
     int_mc = int_mc/((double) n );
     sum_sigma = sum_sigma/((double) n );
     variance=(sum_sigma-int_mc*int_mc);