The eigenvalue problem of singular ergodic control

We consider the problem of finding a real number lambda and a function u satisfying the PDE max{lambda -\Delta u -f,|Du|-1}=0, for all x in R^n. Here f is a convex, superlinear function. We prove that there is a unique lambda* such that the above PDE has a viscosity solution u satisfying u(x)/|x|->1 as |x| tends to infinity. Moreover, we show that associated to lambda^* is a convex solution u^* with D^2u^* uniformly bounded and give two min-max formulae for lambda^. lambda^ has a probabilistic interpretation as being the least, long-time averaged (“ergodic”) cost for a singular control problem involving f.