Juhani Karhumäki Wojciech Plandowski Wojciech Rytter On the complexity of decidable cases of the commutation problem of languages. 105-118 2005 337 Theor. Comput. Sci. 1-3 http://dx.doi.org/10.1016/j.tcs.2004.03.073 db/journals/tcs/tcs337.html#KarhumakiPR05