private void Prime_Click(object sender, EventArgs e)
{
  int[] a = new int[10000];
  int n, i, count = 0;
  bool prime;
  listBox1.Items.Clear();
  n = System.Convert.ToInt32(textBox1.Text, 10);
  for (i = 2; i <= n; i++)
  {
     prime = true;
     for (int j = 0; j < count && prime; j++)
     if (i % a[j] == 0) prime = false;
     if (prime)
     {
       a[count] = i;
       listBox1.Items.Add(a[count].ToString());
       count++;
     }
   }
}
