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 ...

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 ...

Variational Quantum Algorithms (VQAs) are prime candidates for near-term quantum advantage, yet training them is known to be NP-hard. Here we prove a conditional undecidability result for noiseless, ...