Alternation Is Strict For Higher-Order Modal Fixpoint Logic

Florian Bruse
(Universität Kassel)

We study the expressive power of Alternating Parity Krivine Automata (APKA), which provide operational semantics to Higher-Order Modal Fixpoint Logic (HFL). APKA consist of ordinary parity automata extended by a variation of the Krivine Abstract Machine. We show that the number and parity of priorities available to an APKA form a proper hierarchy of expressive power as in the modal mu-calculus. This also induces a strict alternation hierarchy on HFL. The proof follows Arnold's (1999) encoding of runs into trees and subsequent use of the Banach Fixpoint Theorem.

In Domenico Cantone and Giorgio Delzanno: Proceedings of the Seventh International Symposium on Games, Automata, Logics and Formal Verification (GandALF 2016), Catania, Italy, 14-16 September 2016, Electronic Proceedings in Theoretical Computer Science 226, pp. 105–119.
Published: 13th September 2016.

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