On two properties of the minimum broadcast time function

Broadcasting is the problem of dissemination of information in which one piece of information needs to be transmitted to a group of individuals connected by an interconnection network. A widely accepted communication model for this problem is the 1-port, constant model, in which a node of the network can transmit the message only to one neighbor at a time, and the transmission time is constant, regardless the length of the message. Finding an optimum strategy for broadcasting under this model, such that this process is accomplished in minimum time, has been proved to be NP-complete for an arbitrary network. If we model the interconnection network as an undirected graph, the minimum broadcast time function associates to each vertex an integer which represents the minimum time necessary to broadcast the information stored in that vertex to all other vertices. The values of the minimum broadcast time function are known for a very restricted class of graphs, mainly regular ones, and very little is known about this function in general. In this paper we explore two new properties of this function. The first property establishes a connection between this function and the behavior of the optimal broadcast schemes. We prove an exact result for trees and we conjecture it for arbitrary graphs. The second property establishes a connection between this function and the density of the graph.