Encounter-Based Approach to Target Search Problems

In this chapter, we present the encounter-based approach to target search problems, in which the diffusive dynamics is described by the joint probability of the position of the particle and the number of its encounters with a given target set. The knowledge of the statistics of encounters allows one to implement various mechanisms of reactions on the target set, beyond conventional reaction schemes.  We formulate this approach for three relevant settings: discrete random walks, Brownian motion with bulk reactions, and reflected Brownian motion with surface reactions. In all the cases, we discuss the advantages of this approach, its recent applications and possible extensions.