Rumour spreading and graph conductance∗ Flavio Chierichetti, Silvio Lattanzi, Alessandro Panconesi fchierichetti,lattanzi,
[email protected] Dipartimento di Informatica Sapienza Universit`adi Roma October 12, 2009 Abstract if its completion time is poly-logarithmic in the size of We show that if a connected graph with n nodes has the network regardless of the source, and that it is slow conductance φ then rumour spreading, also known as otherwise. randomized broadcast, successfully broadcasts a mes- Randomized broadcast has been intensely investi- sage within O(log4 n/φ6) many steps, with high proba- gated (see the related-work section). Our long term bility, using the PUSH-PULL strategy. An interesting goal is to characterize a set of necessary and/or suffi- feature of our approach is that it draws a connection be- cient conditions for rumour spreading to be fast in a tween rumour spreading and the spectral sparsification given network. In this work, we provide a very general procedure of Spielman and Teng [23]. sufficient condition{ high conductance. Our main moti- vation comes from the study of social networks. Loosely 1 Introduction stated, we are looking after a theorem of the form \Ru- mour spreading is fast in social networks". Our result is Rumour spreading, also known as randomized broadcast a good step in this direction because there are reasons or randomized gossip (all terms that will be used as to believe that social networks have high conductance. synonyms throughout the paper), refers to the following This is certainly the case for preferential attachment distributed algorithm. Starting with one source node models such as that of [18].