[HTML][HTML] Counting branches in trees using games

A Carayol, O Serre - Information and Computation, 2017 - Elsevier
Information and Computation, 2017Elsevier
We study finite automata running over infinite binary trees. A run of such an automaton is
usually said to be accepting if all its branches are accepting. In this article, we relax the
notion of accepting run by allowing a certain quantity of rejecting branches. More precisely
we study the following criteria for a run to be accepting:(i) it contains at most finitely (resp.
countably) many rejecting branches;(ii) it contains infinitely (resp. uncountably) many
accepting branches;(iii) the set of accepting branches is topologically “big”. In all situations …
Abstract
We study finite automata running over infinite binary trees. A run of such an automaton is usually said to be accepting if all its branches are accepting. In this article, we relax the notion of accepting run by allowing a certain quantity of rejecting branches. More precisely we study the following criteria for a run to be accepting:(i)
it contains at most finitely (resp. countably) many rejecting branches;
(ii)
it contains infinitely (resp. uncountably) many accepting branches;
(iii)
the set of accepting branches is topologically “big”.
In all situations we provide a simple acceptance game that later permits to prove that the languages accepted by automata with cardinality constraints are always ω-regular. In the case (ii) where one counts accepting branches it leads to new proofs (without appealing to logic) of a result of Beauquier and Niwiński.
Elsevier
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