On Vertical Grammatical Restrictions that Produce an Infinite Language Hierarchy. - Radek Bidlo; Petr Blatný; Alexander Meduna (Workshop on Formal Models, 2007)

Automata with two-sided pushdowns defined over free groups generated by reduced alphabets. - Petr Blatný; Radek Bidlo; Alexander Meduna (Kybernetika, 2007)