A Mechanised Proof of Gödel’s Incompleteness Theorems using Nominal Isabelle

An Isabelle/HOL formalisation of Gödel’s two incompleteness theorems is presented. The work follows Swierczkowski’s detailed proof of the theorems using hereditarily finite (HF) set theory. Avoiding the usual arithmetical encodings of syntax eliminates the necessity to formalise elementary number theory within an embedded logical calculus. The Isabelle formalisation uses two separate treatments of variable binding: the nominal package is shown to scale to a development of this complexity, while de Bruijn indices turn out to be ideal for coding syntax. Critical details of the Isabelle proof are described, in particular gaps and errors found in the literature.Jesse Alama drew my attention to Swierczkowski, the source material for this ´ project. Christian Urban assisted with nominal aspects of some of the proofs, even writing code. Brian Huffman provided the core formalisation of type hf. Dana Scott offered advice and drew my attention to Kirby. Matt Kaufmann and the referees made many insightful comments.This is the author accepted manuscript. The final version is available from Springer at http://link.springer.com/article/10.1007%2Fs10817-015-9322-