Wilson's Theorem 

Wilson's Theorem
1 2 4 6 10 11  12 13  14  15  16 17  18 19  20 
2 1
3 1 2
5 1 2 1 4
7 1 2 6 3 1 6
10 
11 1 2 6 2 10 5 2 5 1 10
12 
13 1 2 6 11 3 5 9 7 11 6 1 12
14  10 
15  0 0
16 
17 1 2 6 7 1 6 8 13 15 14 1 12 3 8 1 16
18  12 
19 1 2 6 5 6 17 5 2 18 9 4 10 16 15 16 9 1 18

If p is prime, then (p-1)! + 1 is divisible by p.