Fibonacci Sequence

0, 1, 1, 2, 3, 5, 8, 13, ...

First two values are 0 and 1, and then on, next values are formed by sum of last two numbers of sequence
value = (value - 1) + (value - 2)

Factorial

0! = 1
1! = 1
2! = 1 * 2 = 2
3! = 1 * 2 * 3 = 6
4! = 1 * 2 * 3 * 4 = 24
5! = 1 * 2 * 3 * 4 * 5 = 120
...