Truthful Cake Sharing

The classic cake cutting problem concerns the fair allocation of a heterogeneous resource among interested agents. In this paper, we study a public goods variant of the problem, where instead of competing with one another for the cake, the agents all share the same subset of the cake which must be chosen subject to a length constraint. We focus on the design of truthful and fair mechanisms in the presence of strategic agents who have piecewise uniform (i.e., approval) utilities over the cake. On the one hand, we show that the leximin solution is excludably truthful (meaning it is truthful when it can block each agent from accessing parts of the cake that the agent does not claim to desire) and moreover maximizes the guaranteed normalized egalitarian welfare among all excludably truthful and position oblivious mechanisms. On the other hand, we demonstrate that the maximum Nash welfare solution is excludably truthful for two agents (as it coincides with leximin in that case) but not in general. We also provide an impossibility result on truthfulness when blocking is not allowed, and adapt notions of representation to our setting.