The Discontinuity Problem

TL;DR

This paper investigates the weakest discontinuous problem in the Weihrauch lattice, revealing its dependence on axiomatic frameworks and introducing a game-theoretic approach to characterize continuity and effective discontinuity.

Contribution

It introduces the discontinuity problem, analyzes its reducibility, and explores how different axiomatic assumptions affect the problem's position in the Weihrauch lattice.

Findings

The discontinuity problem is reducible to effectively discontinuous problems.

The axiomatic framework determines whether the problem is the weakest discontinuous problem.

Wadge games characterize continuity and effective discontinuity of problems.

Abstract

Matthias Schr\"oder has asked the question whether there is a weakest discontinuous problem in the continuous version of the Weihrauch lattice. Such a problem can be considered as the weakest unsolvable problem. We introduce the discontinuity problem, and we show that it is reducible exactly to the effectively discontinuous problems, defined in a suitable way. However, in which sense this answers Schr\"oder's question sensitively depends on the axiomatic framework that is chosen, and it is a positive answer if we work in Zermelo-Fraenkel set theory with dependent choice and the axiom of determinacy AD. On the other hand, using the full axiom of choice, one can construct problems which are discontinuous, but not effectively so. Hence, the exact situation at the bottom of the Weihrauch lattice sensitively depends on the axiomatic setting that we choose. We prove our result using a variant of Wadge games for mathematical problems. While the existence of a winning strategy for player II characterizes continuity of the problem (as already shown by Nobrega and Pauly), the existence of a winning strategy for player I characterizes effective discontinuity of the problem. By Weihrauch determinacy we understand the condition that every problem is either continuous or effectively discontinuous. This notion of determinacy is a fairly strong notion, as it is not only implied by the axiom of determinacy AD, but it also implies Wadge determinacy. We close with a brief discussion of generalized notions of productivity.