Self-similar groups, automatic sequences, and unitriangular   representations
  We study natural linear representations of self-similar groups over finite fields. In particular, we show that if the group is generated by a finite automaton, then obtained matrices are automatic. This shows a new relation between two separate notions of automaticity: groups generated by automata and automatic sequences. We also show that if the group acts on the tree by $p$-adic automorphisms, then the corresponding linear representation is a representation by infinite triangular matrices. We relate this observation with the notion of height of an automorphism of a rooted tree due to L.Kaloujnine. 
R. Grigorchuk, Y. Leonov, V. Nekrashevych, V. Sushchansky
math.GR