By former and recent results the model of reversible deterministic finite automata is well understood. On the other hand, reversible nondeterministic finite automata and their accepted languages have not systematically been considered in the literature. Here it turns out that reversible nondeterministic finite automata (REV-NFAs) are more powerful compared to their reversible deterministic counterparts, but still cannot accept all regular languages. Moreover, we compare the family of languages accepted by REV-NFAs to the language families accepted by deterministic and nondeterministic finite state automata with irreversibility degree k. Besides these results on the computational power of REV-NFAs we consider closure properties of the language family induced by these devices.
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