private void Display_Click(object sender, EventArgs e)
{
  listBox1.Items.Clear();
  int num = Convert.ToInt32(textBox1.Text, 10);
  if (checkBox1.Checked)
    for (int i = 2; i <= num; i++)
    {
      bool prime = true;
      for (int j = 2; j < (int)i / 2 && prime == true; j++)
        if (i % j == 0) prime = false;
      if (prime) listBox1.Items.Add(i.ToString());
     }
   else
   {
     for (int i = 2; i <= num; i++)
     {
       int sum = 0;
       for (int j = 1; j < i; j++)
          if (i % j == 0) sum += j;
       if (sum == i) listBox1.Items.Add(i.ToString());
     }
   }
}
