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Lean 4 is a programming language with a built-in interactive theorem prover. Mathematical definitions and statements are expressed as code, and proofs are verified by Lean’s type checker: a proof is correct if and only if it type-checks, assuming one trusts Lean’s small trusted kernel.

This book uses abstract algebra (groups, rings) and a bit of category-flavored material (path algebras of quivers) as running examples. These topics are rich enough to be interesting, but simple enough to build from scratch.


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