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Introduction Riemann’s theorem on the negligibility of isolated singularities of bounded holomorphic functions implies Casorati-Weierstrass theorem on the behavior of holomorphic functions near essential isolated singularities. To analyse the behavior of holomorphic functions around their non-isolated essential singularities for extending the Picard theorem, function-theoretic null sets had to be studied. Robin constant and logarithmic capacity are conformal invariants which arose in this context. Later it turned out that they are also useful for other purposes. Existence of Evans potentials characterizes the class of Riemann surfaces with “null-boundary”. Construction of functions of Evans type by Nakai has an application to a question in several complex variables.
Let $O_G$ denote the set of Riemann surfaces which do not have Green functions. By generalizing Evans' theorem, it was proved by by Z. Kuramochi [K-1,2,3] that $R\in O_G$ if and only if $R$ admits an Evans potential, which Kuramochi calls Evans-Selberg potential in view of a work of H. Selberg [Sb] as well as [E]\footnote{[Sb] appeared a little later than [E].}. As a background of Kuramochi's work, one can mention [Nv] which initiated complex analysis on open Riemann surfaces by establishing that, given a plane domain $D$, $D\in O_G$ if and only if the Bergman kernel of $D$ is trivial. M. Ohtsuka [Oht] proved that $D\in O_G$ if and only if $D$ admits a nonconstant bounded superharmonic function.
\begin{definition}If $R\notin O_G$, a divergent sequence of point $p_n$ in $R$ is called \textbf{irregular} if there exists $p'\in R$ such that $\liminf_{n\to\infty}{g(p_n,p')}>0$. \end{definition}
Extensions of Picard's theorem have been obtained in order to describe the essential singularities of meromorphic functions near the sets of null logarithmic capacity (cf. [Km]).
The purpose of this section is to give an account for the construction of canonical potential functions on plane domains based on the equivalence of the logarithmic capacity and the transcendental diameter, which can be generalized on Riemann surfaces. Roughly speaking, the notions of logarithmic capacity and transcendental diameter are refinements of the one dimensional Hausdorff measure. Extensions of Picard’s theorem have been obtained in order to describe the essential singularities of meromorphic functions near the sets of null capacity. As a typical result, see [Km] for instance.
[Km] Kametani, S. On Hausdorff ’s measures and generalized capacities with some of their applications to the theory of functions, Jap. Journ. Math., 19 (1944–48), pp. 217-257.
M. Nakai and T. Tada Nagoya Math. J. Vol. 86 (1982), 85-99
Received October 20, 1979. Both of the authors were supported by Grant-in-Aid for Scientific Research, The Japanese Ministry of Education, Science and Culture.
The Picard principle has been a topic of fascination for quite some time. Many of the publications are due to the first author and his collaborators. This is another interesting paper from this productive group. Reviewer: Chung, Lung Ock
2. Basic results on Pn and Cn. For the Grassmannian Gr(r, n) := {r − dimensional linear subspaces of Cn}, ⨿L∈Gr(r,n) L is a vector bundle of rank r, which is called the universal bundle, denoted by U(r, n) → Gr(r, n). If E is a rank r subbundle of B × C n, one has a map x → Ex from B to Gr(r, n), say f, for which E = f∗U(r, n) holds. Gr(1, n+1) is called the projective space and denoted simply by Pn. P1is nothing but the Riemann sphere.
There are three main approaches one can take to studying compact Riemann surfaces. (1) The classical approach, which combines complex analytic function theory, differential geometry and topology of surfaces together. (2) The modern complex analytic manifold theory, which leans heavily on analytic sheaf theory. (3) Algebraic curve theory, since (quite amazingly) every compact Riemann surface is a projective algebraic curve.