Axiomatic quantum electrodynamics: from causality to convexity of effective action

Theory of electromagnetic field, specified by an effective action functional, is considered. The causality condition is imposed in the form of a requirement that the group velocities of propagation of small and soft disturbances over the background of an external constant field should not exceed the speed of light in vacuum. It is shown that these conditions lead, in particular, to a very definite conclusion about the geometry of the local limit of the effective action. Namely, the surface, which is specified in the local limit by the nonlinear Lagrangian, considered as a function of two invariants of the field has positive Gaussian curvature.