Occurrences of a factor w in an infinite uniformly recurrent sequence u can be encoded by over finite alphabet. This is usually denoted du(w) and called the derived to u. If prefix fixed point primitive substitution φ, then Durand's result from 1998, ψ as well. For non-prefix w, only exceptionally. To study this phenomenon we introduce new notion: A set M substitutions said closed under derivat...
