Scattered one-counter languges have rank less than $ω^2$

Abstract

A linear ordering is called context-free if it is the lexicographic ordering of some context-free language and is called scattered if it has no dense subordering. Each scattered ordering has an associated ordinal, called its rank. It is known that scattered context-free (regular, resp.) orderings have rank less than ω ω (ω, resp).In this paper we confirm the conjecture that one-counter languges have rank less than ω 2 .