Sudan function

In the theory of computation, the Sudan function is an example of a function that is recursive, but not primitive recursive. This is also true of the better-known Ackermann function. The Sudan function was the first function having this property to be published.

It was discovered (and published) in 1927 by Gabriel Sudan, a Romanian mathematician who was a student of David Hilbert.

In general, F₁(x, y) is equal to F₁(0, y) + 2^y x.

...
Wikipedia