“FRACTRAN: A Simple Universal Programming Language for Arithmetic”, John H. Conway1987 (computability, computer security; backlinks)⁠:

To play the fraction game corresponding to a given list

f₁, f₂, …, f_k

of fractions and starting integer N, you repeatedly multiply the integer you have at any stage (initially N) by the earliest f_i in the list for which the answer is integral. Whenever there is no such f_i, the game stops.

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