static void mark (long[] a, int l, int r)
        {
            int i_sqrt_max = (int)Math.Sqrt(r*2) / 2 + 2;
            for (int k = 1; k <= i_sqrt_max; k++) //from 1 because multiples of a[0]=2 aren't in the array
            {
                if (a[k] != 0)
                {
                    for (long i = a[k]+k; i < r; i += a[k])
//where do I have to start i (in relation to l,k) to avoid repeating the steps from i to l?
                    {
                        a[i] = 0;
                    }
                }
            }
        }