Maximizing Nash Social Welfare in 2-Value Instances
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We consider the problem of maximizing the Nash social welfare when allocating a set 𝒢 of indivisible goods to a set 𝒩 of agents. We study instances, in which all agents have 2-value additive valuations: The value of every agent i ∈𝒩 for every good j ∈𝒢 is v_ij∈{p,q}, for p,q ∈ℕ, p ≤ q. Maybe surprisingly, we design an algorithm to compute an optimal allocation in polynomial time if p divides q, i.e., when p=1 and q ∈ℕ after appropriate scaling. The problem is -hard whenever p and q are coprime and p ≥ 3. In terms of approximation, we present positive and negative results for general p and q. We show that our algorithm obtains an approximation ratio of at most 1.0345. Moreover, we prove that the problem is -hard, with a lower bound of 1.000015 achieved at p/q = 4/5.
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