# The string of diamonds is nearly tight for rumour spreading

## TL;DR

This paper compares synchronous and asynchronous rumour spreading protocols, establishing bounds on their expected spread times across all graphs, and demonstrates the near-tightness of these bounds with specific graph examples.

## Contribution

It provides a tight bound on the ratio of spread times for the two protocols and characterizes the possible spread time exponents for infinite graph families.

## Key findings

- Bound on the expected spread time ratio: O(n^{1/3} log^{2/3} n)
- Tightness of the bound up to a factor of O(log n)
- Characterization of achievable spread time exponents for graph families

## Abstract

For a rumour spreading protocol, the spread time is defined as the first time that everyone learns the rumour. We compare the synchronous push&pull rumour spreading protocol with its asynchronous variant, and show that for any $n$-vertex graph and any starting vertex, the ratio between their expected spread times is bounded by $O \left({n}^{1/3}{\log^{2/3} n}\right)$. This improves the $O(\sqrt n)$ upper bound of Giakkoupis, Nazari, and Woelfel (in Proceedings of ACM Symposium on Principles of Distributed Computing, 2016). Our bound is tight up to a factor of $O(\log n)$, as illustrated by the string of diamonds graph. We also show that if for a pair $\alpha,\beta$ of real numbers, there exists infinitely many graphs for which the two spread times are $n^{\alpha}$ and $n^{\beta}$ in expectation, then $0\leq\alpha \leq 1$ and $\alpha \leq \beta \leq \frac13 + \frac23 \alpha$; and we show each such pair $\alpha,\beta$ is achievable.