Nondeterminism is essential for reversal-bounded two-way multihead finite automata. - Andrej Bebják; Ivana Stefáneková (Kybernetika, 1988)

Separation of Deterministic, Nondeterministic and Alternating Complexity Classes. - Andrej Bebják; Ivana Stefáneková (Theor. Comput. Sci., 1991)