Description
I will discuss infrared enhancements that are present in four-derivative theories in four dimensions (and presumably also in two-derivative theories in two dimensions). I will illustrate these effects using the example of quadratic gravity. Both the tree-level and one-loop approximations will be discussed. I will point out that these enhancements cannot be captured by running couplings. I will then also discuss running couplings that arise from genuine ultraviolet divergences.