Regular tree languages in low levels of the Wadge Hierarchy
In this article we provide effective characterisations of regular languages of infinite trees that belong to the low levels of the Wadge hierarchy. More precisely we prove decidability for each of the finite levels of the hierarchy; for the class of the Boolean combinations of open sets $BC(Σ_1^0)$ (i.e. the union of the first $ω$ levels); and for the Borel class $Δ_2^0$ (i.e. for the union of the first $ω_1$ levels).
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