Online Matching on $3$-Uniform Hypergraphs
This work extends the classic online matching problem to hypergraphs, providing optimal algorithms and hardness results for a new setting, though the results are incremental as they generalize known techniques from bipartite graphs.
This paper studies the online matching problem on 3-uniform hypergraphs, providing an optimal primal-dual fractional algorithm with a competitive ratio of (e-1)/(e+1) ≈ 0.4621 and proving its optimality via a constructed adversarial instance. For k≥3, a simple integral algorithm achieves a competitive ratio of 1/2 when online nodes have bounded degree.
The online matching problem was introduced by Karp, Vazirani and Vazirani (STOC 1990) on bipartite graphs with vertex arrivals. It is well-known that the optimal competitive ratio is $1-1/e$ for both integral and fractional versions of the problem. Since then, there has been considerable effort to find optimal competitive ratios for other related settings. In this work, we go beyond the graph case and study the online matching problem on $k$-uniform hypergraphs. For $k=3$, we provide an optimal primal-dual fractional algorithm, which achieves a competitive ratio of $(e-1)/(e+1)\approx 0.4621$. As our main technical contribution, we present a carefully constructed adversarial instance, which shows that this ratio is in fact optimal. It combines ideas from known hard instances for bipartite graphs under the edge-arrival and vertex-arrival models. For $k\geq 3$, we give a simple integral algorithm which performs better than greedy when the online nodes have bounded degree. As a corollary, it achieves the optimal competitive ratio of 1/2 on 3-uniform hypergraphs when every online node has degree at most 2. This is because the special case where every online node has degree 1 is equivalent to the edge-arrival model on graphs, for which an upper bound of 1/2 is known.