#Codomain

For example let's say my domain is N, and codomain is "N>x" and x is a element of N, so the codomai. Will be all natural numbers greater then x (aka the input)?

From Reddit - r/learnmath: Lets say I have a function with a domain and codomain that are countably infinite, is there a function that, depending on the index of the input in the domain, will give me an element of the same index in the codomain?