Aceto, L., W. Fokkink, and A. Ingólfsdóttir. “On a Question of Arto Salomaa: The Equational Theory of Regular Expressions over a Singleton Alphabet Is Not Finitely Based”. BRICS Report Series, Vol. 3, no. 36, June 1996, doi:10.7146/brics.v3i36.20018.