Regular poles and $\beta$-numbers in the theory of holomorphic semigroups

We introduce the notion of regular (boundary) poles for infinitesimal generators of semigroups of holomorphic self-maps of the unit disc. We characterize such regular poles in terms of $\beta$-points (i.e. pre-images of values with positive Carleson-Makarov $\beta$-numbers) of the associated semigroup and of the associated K"onigs intertwining function. We also define a natural duality operation in the cone of infinitesimal generators and show that the regular poles of an infinitesimal generator correspond to the regular null poles of the dual generator. Finally we apply such a construction to study radial multi-slits and give an example of a non-isolated radial slit whose tip has not a positive Carleson-Makarov $\beta$-number.