Oscar H. Ibarra Juhani Karhumäki Alexander Okhotin On Stateless Multihead Automata: Hierarchies and the Emptiness Problem. 94-105 2008 LATIN http://dx.doi.org/10.1007/978-3-540-78773-0_9 conf/latin/2008 db/conf/latin/latin2008.html#IbarraKO08