Word-mappings of level 2
Résumé
A sequence of natural numbers is said to have {\em level k}, for some natural integer $k$, if it can be computed by a deterministic pushdown automaton of level $k$ ([Fratani-Sénizergues, APAL, 2006]). We show here that the sequences of level 2 are exactly the rational formal power series over one undeterminate. More generally, we study mappings {\em from words to words} and show that the following classes coincide:\\ - the mappings which are computable by deterministic pushdown automata of level $2$\\ - the mappings which are solution of a system of catenative recurrence equations\\ - the mappings which are definable as a Lindenmayer system of type HDT0L.\\ We illustrate the usefulness of this characterization by proving three statements about formal power series, rational sets of homomorphisms and equations in words.