What is Turing Machine XN+1?

Turing Machine XN+1 is one of the example Turing Machines that Sir Roger Penrose described on page 60 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 XN+1 Turing Machine does is read the number on the tape (in Expanded Binary Notation), add one to it, and display the answer on the tape (again in Expanded Binary Notation).