On the Pursuit of EFX for Chores: Non-existence and Approximations

On the Pursuit of EFX for Chores: Non-existence and Approximations

Vasilis Christoforidis, Christodoulos Santorinaios

Proceedings of the Thirty-Third International Joint Conference on Artificial Intelligence
Main Track. Pages 2713-2721. https://doi.org/10.24963/ijcai.2024/300

We study the problem of fairly allocating a set of chores to a group of agents. The existence of envy-free up to any item (EFX) allocations is a long-standing open question for both goods and chores. We resolve this question by providing a negative answer for the latter, presenting a simple construction that admits no EFX solutions for allocating six items to three agents equipped with superadditive cost functions, thus proving a separation result between goods and bads. In fact, we uncover a deeper insight, showing that the instance has unbounded approximation ratio. Moreover, we show that deciding whether an EFX allocation exists is NP-complete. On the positive side, we establish the existence of EFX allocations under general monotone cost functions when the number of items is at most n + 2. We then shift our attention to additive cost functions. We employ a general framework in order to improve the approximation guarantees in the well-studied case of three additive agents, and provide several conditional approximation bounds that leverage ordinal information.
Keywords:
Game Theory and Economic Paradigms: GTEP: Fair division