Many universal models of computation are known: Turing machines, tag-systems, cellular
automata, Diophantine equations, etc. (see [35]). Of course, any universal model can simulate
and be simulated by any other universal model. But it is Collatz-like functions, and not
another model, that appear naturally in this study. Their unexpectedly pervasive presence
leads to wonder about the significance of their status among mathematical beings.