Diamonds and Dominoes: Impossibility Results for Associative Modal Logics

We show that for any class of Boolean algebras with an associative operator, if it contains the complex algebra of (P(N), U), its equational theory is undecidable. Equivalently, any associative normal modal logic valid over the frame (P(N), U) is undecidable. This settles a long-open question on the decidability of hyperboolean modal logic (Goranko and Vakarelov, 1999), and addresses several related problems.