3-Majority and 2-Choices with Many Opinions

Abstract: We present the first nearly-optimal bounds on the consensus time for the well-known synchronous consensus dynamics, specifically 3-Majority and 2-Choices, for an arbitrary number of opinions. In synchronous consensus dynamics, we consider an $n$-vertex complete graph with self-loops, where each vertex holds an opinion from ${1,\dots,k}$. At each discrete-time round, all vertices update their opinions simultaneously according to a given protocol. The goal is to reach a consensus, where all vertices support the same opinion. In 3-Majority, each vertex chooses three random neighbors with replacement and updates its opinion to match the majority, with ties broken randomly. In 2-Choices, each vertex chooses two random neighbors with replacement. If the selected vertices hold the same opinion, the vertex adopts that opinion. Otherwise, it retains its current opinion for that round. Improving upon a line of work [Becchetti et al., SPAA'14], [Becchetti et al., SODA'16], [Berenbrink et al., PODC'17], [Ghaffari and Lengler, PODC'18], we prove that, for every $2\le k \le n$, 3-Majority (resp.\ 2-Choices) reaches consensus within $\widetilde{\Theta}(\min{k,\sqrt{n}})$ (resp.\ $\widetilde{\Theta}(k)$) rounds with high probability. Prior to this work, the best known upper bound on the consensus time of 3-Majority was $\widetilde{O}(k)$ if $k \ll n^{1/3}$ and $\widetilde{O}(n^{2/3})$ otherwise, and for 2-Choices, the consensus time was known to be $\widetilde{O}(k)$ for $k\ll \sqrt{n}$.