Unambiguous separators for tropical tree automata

TL;DR

This paper proves that for max-plus and min-plus automata over trees with real weights, an unambiguous automaton can be effectively constructed to separate and approximate the functions, generalizing previous word-based results.

Contribution

It extends the unambiguous automaton construction to tree automata and establishes effective separation between functions, generalizing prior word-based results.

Findings

Existence of unambiguous tropical automaton between given max-plus and min-plus automata.

Generalization from word automata to tree automata.

Effective construction of separating automaton.

Abstract

In this paper we show that given a max-plus automaton (over trees, and with real weights) computing a function $f$ and a min-plus automaton (similar) computing a function $g$ such that $f\leqslant g$, there exists effectively an unambiguous tropical automaton computing $h$ such that $f\leqslant h\leqslant g$. This generalizes a result of Lombardy and Mairesse of 2006 stating that series which are both max-plus and min-plus rational are unambiguous. This generalization goes in two directions: trees are considered instead of words, and separation is established instead of characterization (separation implies characterization). The techniques in the two proofs are very different.