INFINITE-DIMENSIONAL HAMILTON--JACOBI EQUATIONS FOR STATISTICAL INFERENCE ON SPARSE GRAPHS\ast

We study the well-posedness of an infinite-dimensional Hamilton-Jacobi equation posed on the set of nonnegative measures and with a monotonic nonlinearity. Our results will be used in a companion work to propose a conjecture and prove partial results concerning the asymptotic mutual information in the assortative stochastic block model in the sparse regime. The equation we consider is naturally stated in terms of the Gateaux derivative of the solution, unlike previous works in which the derivative is usually of transport type. We introduce an approximating family of finitedimensional Hamilton--Jacobi equations and use the monotonicity of the nonlinearity to show that no boundary condition needs to be prescribed to establish well-posedness. The solution to the infinitedimensional Hamilton--Jacobi equation is then defined as the limit of these approximating solutions. In the special setting of a convex nonlinearity, we also provide a Hopf--Lax variational representation of the solution.