https://www2.karlin.mff.cuni.cz/~holub/
This is an unofficial reimagining of the Archive of Formal Proofs
Lyndon wordsby Štěpán Holub and Štěpán Starosta
Graph Lemmaby Štěpán Holub and Štěpán Starosta
Combinatorics on Words Basicsby Štěpán Holub, Martin Raška and Štěpán Starosta