A pumping-like lemma for languages over infinite alphabets

Abstract

We prove a pumping-like lemma for languages accepted by one-register alternating finite-memory automata over an infinite alphabet. Every sufficiently long accepted word has a nonempty factor in a bounded suffix that can be repeated under successive powers of a single alphabet permutation. The proof analyzes accepting runs backwards, counting register values by their state profiles within the remaining suffix and summarizing values outside it. We derive effective semi-linearity of the sets of accepted word lengths and prove non-closure under reversal, concatenation, and Kleene star. We also show that this pumping lemma cannot be extended to alternating automata with two registers.