A no-go theorem for regular black holes

In this article we discuss a no-go theorem for generating regular black holes from a Lagrangian theory. We prove that the general solution has always a Schwarzschild-like term $c/r$, as long as the matter Lagrangian depends neither on the metric, nor on its derivatives; we also prove that, under suitable additional conditions, these two conditions are also equivalent to $g_{00}g_{11} = -1$. Finally, we prove that $c/r$ is the only non-Lagrangian singularity eventually present into the solution.