[Turing36] A. Turing, On Computable Numbers, with an Application to the Entscheidungsproblem, London Math Soc. Ser 2. Vol 42,Nov 12,1936,230-265. [This is the famous paper that shows that any problem that can be solved by any general purpose computer can also be solved by any other general purpose computer, given enough time and space. It also shows that a large class of problems can never be solved by a computer. The so-called Halting problem, in particular, is proven unsolvable.]