Name of a sequence that is unknown to stop on a given input

Question

Several years ago I was self learning lisp by a book which I do not remember. As an example there was a recursively defined sequence which was said to be unknown to be finite or not for every input --- to generate a sequence you were required to provide two(?) initial values. At least that is how I remember.

There was a Wikipedia article on it but I can't find it either. Does anyone know what I'm talking about?

Answer 1

It has many names, I first heard about it as the Syracuse problem, but the most prominent name is maybe Collatz Conjecture.