This is the old version! The new formalisation lives at: https://github.com/Bergschaf/lean-banach-tarski
The Banach-Tarski theorem states the following: Given a sphere in 3D space, you are able to obtain an identical copy of the sphere by decomposing it into a finite number of subsets, rotating them and putting them back together. This repo contains the project to formalise the Banach-Tarksi theorem in Lean4.