The pattern goes: Start with 1 Then follow these four rules over and over Square it. Get 1 Double it. Get 2 Double it. Get 4 Add 1. Get 5 Square it. Get 25 Double it. Get 50 Double it. Get 100 Add 1. Get 101 Square it. Get 10201 Double it. Get 104060401 Double it. Get 208129802 Add 1. Get 208120803 Answer: the next one after 25 was 50. [This pattern is as good as any.] Edwin