Ahlfors covering surface theory

This theory compares the area, boundary length and Euler characteristic of a surface mapped holomorphically to another surface, using the pulled-back metric. It extends covering-degree and Riemann-Hurwitz formula estimates when some boundary maps into the interior of the target. The Ahlfors second fundamental theorem and Ahlfors five islands theorem are important consequences.

For a nonconstant meromorphic function on the plane, let $S(R)$ be its normalized pulled-back spherical area on $|z|<R$, and $L(R)$ the spherical length of its boundary image counted with multiplicity. The Cauchy-Schwarz inequality gives $L(R{)}^2\le 8{\pi }^{2}R{S}^{\prime }(R)$. If $L/S$ stayed above a positive constant for all large $R$, integration of $(1/S{)}^{\prime }\le -c/R$ would make $1/S$ negative. Thus there are arbitrarily large radii with $L/S$ arbitrarily small, even when the total area is finite.

For a finite simply connected covering surface over the Riemann sphere and $q\ge 3$ fixed separated Jordan domains, let $n_j$ count its simply connected islands over the $j$th domain once each, regardless of mapping degree. Let $S$ be its average sheet number and $L$ its spherical boundary length. The displayed inequality holds with $h$ depending only on the target domains. For regular bordered surfaces $f:X\to Y$, a general formulation is $\max \{-\chi (X),0\}\ge -S\chi (Y)-hL$, where $L$ counts only the boundary mapped into the interior of $Y$ and $\chi$ is the modern Euler characteristic. The boundary error disappears for an actual branched covering. The covering-surface formulation is explained in www.math.purdue.edu/~eremenko/dvi/ahl.pdf ; the simply connected island estimate is also recorded in Lemma 2 of pdfs.semanticscholar.org/d247/562607e237af7c8fc6e683b77be6b94829fa.pdf .

Every nonconstant meromorphic function on the complex plane has a simple island over at least one of any five Jordan domains on the Riemann sphere with disjoint closures. The Ahlfors second fundamental theorem, area comparison over target domains, and a length-area exhaustion of the complex plane prove this: without a simple island every counted island has degree at least two, forcing $3S\le (5/2)S+O(L)$.

An island over a Jordan domain $D$ is a relatively compact component $U$ of its inverse image on which the meromorphic function is a proper map onto $D$. A component reaching the boundary of the source is instead a peninsula. The island has an integer mapping degree; degree one means that the map is a biholomorphism. A simply connected island can have larger degree and need not be a simple island.

A simple island is an island of a meromorphic function of mapping degree one. It is mapped biholomorphically onto the target Jordan domain. This is stronger than the source island being simply connected.

For a holomorphic map between metric Riemann surfaces, the average sheet number is the pulled-back source area divided by the target area. The area formula identifies it with the average number of preimages counted with multiplicity. The same definition over a target region $D$ gives $S_D=A_f(f^{-1}D)/A(D)$.