Tight Distributed Sketching Lower Bound for Connectivity

In this paper, we study the distributed sketching complexity of connectivity. In distributed graph sketching, an $n$-node graph $G$ is distributed to $n$ players such that each player sees the neighborhood of one vertex. The players then simultaneously send one message to the referee, who must compute some function of $G$ with high probability. For connectivity, the referee must output whether $G$ is connected. The goal is to minimize the message lengths. Such sketching schemes are equivalent to one-round protocols in the broadcast congested clique model. We prove that the expected average message length must be at least $Ω(\log^3 n)$ bits, if the error probability is at most $1/4$. It matches the upper bound obtained by the AGM sketch [AGM12], which even allows the referee to output a spanning forest of $G$ with probability $1-1/\mathrm{poly}\, n$. Our lower bound strengthens the previous $Ω(\log^3 n)$ lower bound for spanning forest computation [NY19]. Hence, it implies that connectivity, a decision problem, is as hard as its "search" version in this model.