“FRACTRAN is a simple theoretical programming language for arithmetic that has none of the defects described above.
The FRACTRAN User Manual

What is FRACTRAN? FRACTRAN is a very simple Turing-complete programming language invented by John Conway. The program consists of a list of fractions. The current state of the prgram is a positive integer. Each step, the state is multiplied by the first fraction such that the result is a whole number.

 (Using mouse to drag around a fraction to move/re-order, or drag/drop to garbage/delete bin to remove it.)

Or, load a pre-defined program:

Or, load a program from a file:

 (Please input a integer value for N below)

Save n

A note about file formats The starting index is separated from the program by a semicolon. The starting index may be left blank. The program’s fractions are separated by commas, and the numerator and denominator are separated by a slash (ex. 144/77). Save it as a .frac, .fc, .fractran, or .txt file.