### Abstract

A problem posed by Aldous is to estimate the complexity of finding a (1 - epsilon)-optimal particle in a branching random walk. This is computed in terms of the probability of existence of a trajectory staying forever above the critical drift minus epsilon. (it is known that no particle can stay above the critical drift forever). I will then discuss the computation of this probability, in a continous time (branching Brownian motion) setting, which involves estimating solutions to the KPP equation.