The Lean theorem prover is a proof assistant developed principally by Leonardo de Moura at Microsoft Research.

The Lean mathematical library, *mathlib*, is a community-driven effort
to build a unified library of mathematics formalized in the
Lean proof assistant. The library also contains definitions
useful for programming. This project is very active, with many
regular contributors and daily activity.

The contents, design, and community organization of mathlib are described in the paper The Lean mathematical library, which appeared at CPP 2020. You can get a bird's eye view of what is in the library by reading the library overview. You can also have a look at our repository statistics to see how it grows and who contributes to it.

You can try Lean in your web browser, install it in an isolated folder, or go for the full install. Lean is free, open source software. It works on Linux, Windows, and MacOS.

You can learn by playing a game, following tutorials, or reading books.

- Learning resources
- Theorem Proving in Lean (an introduction)
- API documentation of mathlib

Lean has very diverse and active community. It gathers mostly on a Zulip chat and on GitHub. You can get involved and join the fun!

A *proof assistant* is a piece of software that provides a language
for defining objects, specifying properties of these objects,
and proving that these specifications hold.
The system checks that these proofs are correct down to their logical foundation.

These tools are often used to verify the correctness of programs. But they can also be used for abstract mathematics, which is something of interest to the mathlib community. In a formalization, all definitions are precisely specified and all proofs are virtually guaranteed to be correct.

In 2016, Ellenberg and Gijswijt established a new upper bound on the size of subsets of $𝔽^n_q$ with no three-term arithmetic progression. This problem has received much mathematical attention, particularly in the case $q = 3$, where it is commonly known as the cap set problem. Ellenberg and Gijswijt's proof was published in the Annals of Mathematics and is noteworthy for its clever use of elementary methods.

Perfectoid spaces are sophisticated objects in arithmetic geometry introduced by Peter Scholze in 2012. We formalised enough definitions and theorems in topology, algebra and geometry to define perfectoid spaces in the Lean theorem prover. This experiment confirms that a proof assistant can handle complexity in that direction, which is rather different from formalising a long proof about simple objects.

The continuum hypothesis states that there is no cardinality between the smallest infinite cardinal and the cardinality of the continuum. It was posed by Cantor in 1878 and was the first problem on Hilbert’s list of twenty- three unsolved problems in mathematics. Gödel and Cohen proved this is independent, i.e. neither provable nor disprovable, from ZFC.

A broad class of data types, including arbitrary nestings of inductive types, coinductive types, and quotients, can be represented as quotients of polynomial functors. This provides perspicuous ways of constructing them and reasoning about them in an interactive theorem prover.

You can find many more in the list of papers.