Strong Approximate Consensus Halving and the Borsuk-Ulam Theorem

Abstract

In the consensus halving problem we are given n agents with valuations over the interval [0, 1]. The goal is to divide the interval into at most n + 1 pieces (by placing at most n cuts), which may be combined to give a partition of [0, 1] into two sets valued equally by all agents. The existence of a solution may be established by the Borsuk-Ulam theorem. We consider the task of computing an approximation of an exact solution of the consensus halving problem, where the valuations are given by distribution functions computed by algebraic circuits. Here approximation refers to computing a point that ε-close to an exact solution, also called strong approximation. We show that this task is polynomial time equivalent to computing an approximation to an exact solution of the Borsuk-Ulam search problem defined by a continuous function that is computed by an algebraic circuit.The Borsuk-Ulam search problem is the defining problem of the complexity class BU. We introduce a new complexity class BBU to also capture an alternative formulation of the Borsuk-Ulam theorem from a computational point of view. We investigate their relationship and prove several structural results for these classes as well as for the complexity class FIXP.

The Complexity of Necklace Splitting, Consensus-Halving, and Discrete Ham Sandwich

“…The authors also show that the exact version lies in the newly introduced computational class BU, which captures the complexity of the exact Borsuk-Ulam problem and conjecture that it is actually BU-complete. Batziou, Hansen, and Høgh [8] very recently proved that finding a strong approximation of the problem is complete for the appropriate counterpart of BU, the class BU α .…”