Embedding and factoring theorems, absolute retracts and the map extension property for multidimensional subshifts



Subshifts of finite type are central objects in symbolic dynamics. In the classical one-dimensional case, namely subshifts of finite over the group of integers Z , important structural results have been known for decades (although other basic problems remain wide open). Any Z-subshift of finite type decomposes into irreducible components and wandering points. Any irreducible SFT becomes topologically mixing after passing to some power of the shift. Krieger's embedding theorem provides (essentially) decidable necessary and sufficient conditions for an arbitrary subshift to embed in a given topologically mixing SFT. Boyle's factor theorems give (essentially) decidable for factoring between mixing SFTs. The situation for multidimensional subshifts is far less structured and far more mysterious. It is well-known that multidimensional subshifts of finite type can exhibit a wild variety of "pathological behavior". One is soon faced with undecidability issues, and there seems to be little hope to obtain a tractable structure theory in complete generality. Over the years various properties of multidimensional subshifts have been introduced and studied, in an attempt to recover and generalize some structural aspects of the one-dimensional theory (eg “square mixing”, “block gluing”, “strong irreducibility”, “topological strong spatial mixing”, “the finite extension property”). Lightwood has obtained a partial extension of Krieger's embedding theorem for square-filling mixing square-filling mixing Zˆ2-subshifts of finite type. Briceno, McGoff and Pavlov have obtained a partial extension of Boyle's lower entropy theorem for Zˆd-subshifts of finite type with the finite extension property. In this talk I will describe new (and in a suitable "sensecomplete”) multidimensional generalizations of both Krieger's embedding theorem and of Boyle's lower entropy factor theorem. The formulation of these results involves the introduction of a new and seemingly fundamental property of some multidimensional subshifts: The map extension property, introduced implicitly by Mike Boyle in the early 1980's for Z-subshifts. This new property also turns out to be the natural adaptation of the notion of an absolute retract, introduced by Borsuk in the 1930's, to the category of subshifts.
Date made available27 Feb 2024

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