  We introduce MLFMF, a collection of data sets for benchmarking recommendation
systems used to support formalization of mathematics with proof assistants.
These systems help humans identify which previous entries (theorems,
constructions, datatypes, and postulates) are relevant in proving a new theorem
or carrying out a new construction. Each data set is derived from a library of
formalized mathematics written in proof assistants Agda or Lean. The collection
includes the largest Lean~4 library Mathlib, and some of the largest Agda
libraries: the standard library, the library of univalent mathematics
Agda-unimath, and the TypeTopology library. Each data set represents the
corresponding library in two ways: as a heterogeneous network, and as a list of
s-expressions representing the syntax trees of all the entries in the library.
The network contains the (modular) structure of the library and the references
between entries, while the s-expressions give complete and easily parsed
information about every entry. We report baseline results using standard graph
and word embeddings, tree ensembles, and instance-based learning algorithms.
The MLFMF data sets provide solid benchmarking support for further
investigation of the numerous machine learning approaches to formalized
mathematics. The methodology used to extract the networks and the s-expressions
readily applies to other libraries, and is applicable to other proof
assistants. With more than $250\,000$ entries in total, this is currently the
largest collection of formalized mathematical knowledge in machine learnable
format.
