Pen on printer paper.
 
Alan Turing is one of the greatest mathematicians of all time.
 
Further reading: 
 
https://www.genealogy.math.ndsu.nodak.edu/id.php?id=8014
 
http://www-history.mcs.st-and.ac.uk/Biographies/Turing.html
 
https://en.wikipedia.org/wiki/Alan_Turing