Category Archives: combinatorics

## Counting Bijective, Injective, and Surjective Functions

In this post we'll give formulas for the number of bijective, injective, and surjective functions from one finite set to another. Although it's not difficult, a formula for the number of surjective functions was one of the first problems I solved as an undergrad that got me interested in recurrence relations and combinatorics. Let's use […]