On Allocating Goods to Maximize Fairness

We consider the Max-Min Allocation problem: given a set A of m agents and a set I of n items, where agent A is an element of A has utility u(A,i) for item i is an element of I, our goal is to allocate items to agents so as to maximize fairness. Specifically, the utility of an agent is the sum of its utilities for the items it receives, and we seek to maximize the minimum utility of any agent. While this problem has received much attention recently, its approximability has not been well-understood thus far: the best known approximation algorithm achieves an (O) over tilde(root m)-approximation, and in contrast, the best known hardness of approximation stands at 2. Our main result is an algorithm that achieves an (O) over tilde (n(epsilon))-approximation for any epsilon = Omega(log log n/log n) in time n(O(1/epsilon)). In particular, we obtain poly-logarithmic approximation in quasi-polynomial time, and for every constant epsilon > 0, we obtain an (O) over tilde (n(epsilon))-approximation in polynomial time. An interesting technical aspect of our algorithm is that we use as a building block a linear program whose integrality gap is Omega(root m). We bypass this obstacle by iteratively using the solutions produced by the LP to construct new instances with significantly smaller integrality gaps, eventually obtaining the desired approximation. As a corollary of our main result, we also show that for any constant epsilon > 0, an O(m(epsilon))-approximation can be achieved in quasi-polynomial time. We also investigate the special case of the problem, where every item has non-zero utility for at most two agents. This problem is hard to approximate up to any factor better than 2. We give a factor 2-approximation algorithm.