Prophet Inequalities Beyond Utilitarian Social Welfare
In the classical i.i.d. prophet-inequality problem, a single item is allocated to one of $n$ agents who arrive sequentially, with values drawn independently from a known distribution. When an agent arrives, their value is revealed, and the algorithm must immediately allocate the item or continue. The usual objective is utilitarian welfare: the expected value of the recipient. Guarantees for this objective extend to allocating $m$ indivisible items to sequentially arriving agents with i.i.d.\ additive values. Utilitarian welfare, however, ignores how expected utility is distributed across agents. Motivated by a rich literature in fair division, we instead evaluate an online rule by its generalized $p$-mean welfare, which includes utilitarian welfare at $p=1$, Nash welfare (the geometric mean of utilities) at $p=0$, and egalitarian welfare (the minimum utility) as $p\to-\infty$. When the number of items is large, we show that this many-item fair-division problem is captured exactly by a single-item prophet problem evaluated by the $p$-mean of agents' expected utilities. We characterize this single-item problem: every online rule is weakly Pareto dominated by a quantile-threshold rule, and an optimal egalitarian rule equalizes agents' expected utilities. We prove that for every $n$, the online optimum is at least $Γ\approx0.7059$ times the prophet's egalitarian welfare; by monotonicity of generalized means, the same guarantee holds for every $p\le1$. Further, for egalitarian welfare, the optimal ratio converges to $Γ$ as $n\to\infty$. Thus, asymptotically, optimizing egalitarian rather than utilitarian welfare costs only about four percentage points relative to the classical utilitarian ratio of $0.7451$. Finally, when $m=n$, the worst-case competitive ratio converges to zero as $n\to\infty$ for every $p\le0$, showing that the large-item assumption is necessary.