Janusz A. Brzozowski Elyot Grant Jeffrey Shallit Closures in Formal Languages and Kuratowski's Theorem. 125-144 2009 Developments in Language Theory http://dx.doi.org/10.1007/978-3-642-02737-6_10 conf/dlt/2009 db/conf/dlt/dlt2009.html#BrzozowskiGS09