What is Turing Machine UNx2?

Turing Machine UNx2 is one of the example Turing Machines that Sir Roger Penrose described on page 55 of his book The Emperor's New Mind (Oxford University Press, 1989).

It is also one of the sample Turing Machines that's built into both of the Excel files that you can download from the Home page of this website.

What the UNx2 Turing Machine does is read the number on the tape (in Unary Notation), double it, and display the answer on the tape (again in Unary Notation).