Two dimensional wave--Klein-Gordon equations with semilinear nonlinearities

From the work on the weak-null condition by Lindblad and Rodnianski, it is well-known that bad' quadratic sourcing terms are allowed to appear in coupled semilinear wave equations in three spatial dimensions, provided that such terms appear as sources for good’ variables and that the good variables feed back into the system via `good’ sourcing terms. Motivated by these ideas, in this paper we investigate the small data global existence and pointwise decay of solutions to two systems of coupled wave–Klein-Gordon equations in two spatial dimensions. In particular, we consider critical semilinear nonlinearities for the wave equation and below-critical semilinear nonlinearities for the Klein-Gordon equation. An interesting feature of our two systems is that if the nonlinearities of our PDEs were to be swapped, the nonlinear term in the wave equation would lead to finite-time blow-up.