On Finding Balanced Bicliques via Matchings

In the Maximum Balanced Biclique Problem (MBB), we are given an n-vertex graph G= (V, E), and the goal is to find a balanced complete bipartite subgraph with q vertices on each side while maximizing q. The MBB problem is among the first known NP-hard problems, and has recently been shown to be NP-hard to approximate within a factor n^{1 - o ( 1 )}, assuming the Small Set Expansion hypothesis [Manurangsi, ICALP 2017]. An O(n/ log n) approximation follows from a simple brute-force enumeration argument. In this paper, we provide the first approximation guarantees beyond brute-force: (1) an O(n/ log ²n) efficient approximation algorithm, and (2) a parameterized approximation that returns, for any r∈ N, an r-approximation algorithm in time exp(O(nrlogr)). To obtain these results, we translate the subgraph removal arguments of [Feige, SIDMA 2004] from the context of finding a clique into one of finding a balanced biclique. The key to our proof is the use of matching edges to guide the search for a balanced biclique.