Expressive Power of Pebble Automata
نویسندگان
چکیده
Two variants of pebble tree-walking automata on trees are considered that were introduced in the literature. It is shown that for each number of pebbles, the two models have the same expressive power both in the deterministic case and in the nondeterministic case. Furthermore, nondeterministic (resp. deterministic) treewalking automata with n+ 1 pebbles can recognize more languages than those with n pebbles. Moreover, there is a regular tree language that is not recognized by any treewalking automaton with pebbles. As a consequence, FO+posTC is strictly included in MSO over trees.
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تاریخ انتشار 2006