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arXiv · 2605.21448

A Note on EFX Inapproximability for Chores

Vasilis Christoforidis
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Abstract

We study the approximability of EFX allocations for indivisible chores under complement-free cost functions. The non-existence of exact EFX allocations for general monotone functions for chores is known from \cite{CS24}, and a result of \cite{akrami2026} transfers such comparison-based non-existence results to monotone submodular, and hence subadditive, functions. We strengthen this picture by giving explicit constant-factor inapproximability results for submodular and subadditive functions. Our main construction is a three-agent, six-chore instance with monotone subadditive cost functions for which no $\alpha$-EFX allocation exists for any $1\le \alpha<2^{1/3}\approx 1.26$, thus narrowing the gap with the known upper bound of $2$. The construction is obtained by refining the original counterexample of \cite{CS24} and using the approach of \cite{mackenzie2026}. We also give a weighted-coverage realization of the ordinal profile, yielding an instance in which no $\alpha$-EFX allocation exists for any $1\le \alpha<20/19$ under submodular costs. Thus, even within well-studied complement-free classes, EFX for chores admits nontrivial constant lower bounds on approximability.

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Vasilis Christoforidis. 2026-05-20. A Note on EFX Inapproximability for Chores. https://arxiv.org/abs/2605.21448

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