Im trying to find a general formula I can store in my calculator that can find the number of onto (surjective) functions exists for a relation of when M is mapped to N. I can't seem to find a nice formula for it, but based on the below material I will show you what I have developed. From: Discrete Mathemeatics and Its Applications 7th Edition Rosen pg. 561 From: Discrete Mathemeatics and Its Applications 7th Edition Rosen pg. 512 Random Chegg Problem from Google Search I'm using to verify with: Here is what I've formed, written out: In the TI nSpire CX CAS, it's written like this: And as you can see, the chegg problem I'm trying to verify it with doesn't match! But I think it should! If I manually iterate the function like so: The values are correct and what Chegg is showing. What have I done wrong in my equation?