  We consider a setting in which a single item of content (such as a song or a
video clip) is disseminated in a population of mobile nodes by opportunistic
copying when pairs of nodes come in radio contact. We propose and study models
that capture the joint evolution of the population of nodes interested in the
content (referred to as destinations), and the population of nodes that possess
the content. The evolution of interest in the content is captured using an
influence spread model and the content spread occurs via epidemic copying.
Nodes not yet interested in the content are called relays; the influence spread
process converts relays into destinations. We consider the decentralized
setting, where interest in the content and the spread of the content evolve by
pairwise interactions between the mobiles. We derive fluid limits for the joint
evolution models and obtain optimal policies for copying to relay nodes in
order to deliver content to a desired fraction of destinations. We prove that a
time-threshold policy is optimal while copying to relays. We then provide
insights into the effects of various system parameters on the co-evolution
model through simulations.
