An SFT (shift of finite type) is a set of maps to some finite alphabet that is defined by a finite number of disallowed finite words.


By simple I mean has a small alphabet and a small number of disallowed words.



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> **QUESTION.** What are some of the simplest SFTs on $\mathbb{Z}^2$ that have no periodic points?
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What is the simplest SFT on $\mathbb{Z}^2$ that has no periodic points?
Wang tiles are unit squares with edges marked with colors, and the problem of whether a given set of Wang tiles can tile the plane such that edges of adjacent squares match has been studied exhaustively (see <https://en.wikipedia.org/wiki/Wang_tile>). In particular, there is a set of 11 Wang tiles using four colors which tiles the plane, but there is no periodic tiling using them, and this example is essentially minimal for Wang tilings. 


This provides a $\mathbb{Z}^2$ SFT whose alphabet has 11 symbols, and with forbidden words that are pairs of adjacent (vertically or horizontally) tiles whose edges do not match.


I don't know if this qualifies as being "simple", but as these things go it's not too bad: the original construction of a set of Wang tiles with this property had 20,426 tiles in it.


