This is a factorial sequence that can be modeled by An = n!. As you may see, the increasing numbers are factorials of 1,2,3,4, and 5. Factorial means multiplying backwards and is represented by !. For example, 1! is 1*1 =1. 2! is 2*1, 3! is 3*2*1, 4! is 4*3*2*1 etc.
