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### Abstract

This entry provides a formalisation of Niven's famously short one-page proof that $\pi$ is irrational. The proof uses only elementary algebra and analysis.

The intrinsic de Bruijn factor, i.e. the file size ratio between the gzipped Isabelle sources and a gzipped LaTeX version of the original paper's content, is roughly 4 despite the original paper's terse presentation.