One might also find it interesting the relationship of Combinations (_{n}C_{r}), and Pascal's Triangle. For each row, *n* of the triangle (top row being 0), each element along that row, *r*, is == _{n}C_{r}.

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1 <-- n=4
Combinations of 0: 1
1: empty set
Combinations of 1: 4
1: A
2: B
3: C
4: D
Combinations of 2: 6
1: AB
2: AC
3: AD
4: BC
5: BD
6: CB
Combinations of 3: 4
1: ABC
2: ABD
3: ACD
4: BCD
Combinations of 4: 1
1: ABCD