How to Cut a Discrete Cake Fairly

Abstract

Cake-cutting is a fundamental model of dividing a heterogeneous resource, such as land, broadcast time, and advertisement space. In this study, we consider the problem of dividing indivisible goods fairly under the connectivity constraints of a path. We prove that a connected division of indivisible items satisfying a discrete counterpart of envy-freeness, called envy-freeness up to one good (EF1), always exists for any number of agents n with monotone valuations. Our result settles an open question raised by Bilò et al. (2019), who proved that an EF1 connected division always exists for four agents with monotone valuations. Moreover, the proof can be extended to show the following (1) ``secretive" and (2) ``extra" versions: (1) for n agents with monotone valuations, the path can be divided into n connected bundles such that an EF1 assignment of the remaining bundles can be made to the other agents for any selection made by the “secretive agent”; (2) for n+1 agents with monotone valuations, the path can be divided into n connected bundles such that when any ``extra agent” leaves, an EF1 assignment of the bundles can be made to the remaining agents.