Mohamed Faouzi Atig Benedikt Bollig Peter Habermehl Emptiness of Multi-pushdown Automata Is 2ETIME-Complete. 121-133 2008 Developments in Language Theory http://dx.doi.org/10.1007/978-3-540-85780-8_9 conf/dlt/2008 db/conf/dlt/dlt2008.html#AtigBH08