(Phys.org) -- As a testament to how differently things work in the quantum and classical regimes, physicists have found that a problem that is easily solved in a classical context cannot be solved at ...

Abstract: The purpose of this paper is to gain a better understanding of the structure of undecidable problems in automata theory by investigating the degree of unsolvability of these problems. This ...

Abstract: This paper describes some general results about hierarchies of undecidable problems in automata theory, and studies how properties of sets accepted by automata change from decidable to ...

The Post Correspondence Problem (PCP) is an undecidable problem. It asks whether a set of domino-like pairs of strings can be arranged in a sequence so that the top and bottom strings match. There's ...