Ahlfors theory noun1 •A geometric counterpart to Nevanlinna theory that extends the applicability of the concept of covering surface (of a topological space) by defining a covering number (a generalised "degree of covering") applicable to any bordered Riemann surface equipped with a conformal Riemannian metric.In this chapter we use Ahlfors' theory of covering surfaces to obtain results on the functional n(r,a). — 1968
