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Math 411: Honours Complex Variables - University of Alberta

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118 CHAPTER 17. THE RIEMANN MAPPING THEOREM<br />

Condition (xi) makes no reference to holomorphic functions and is entirely topological<br />

in nature. It means that there is a bijective, continuous map f: D → D with<br />

a continuous inverse. Since (x) is preserved under homeomorphisms, we see that (xi)<br />

implies (x). For the converse, it is sufficient to show that C is homeomorphic to D<br />

(for D �= C, this is clear by Theorem 17.3). Since<br />

and<br />

C → D, z ↦→ z<br />

1+|z|<br />

D → C, z ↦→ z<br />

1−|z|<br />

are continuous and inverse to each other, this is indeed the case.<br />

The converse to Problem 11.1 states that the property<br />

(xii) for every holomorphic function f: D → C, there exists a sequence <strong>of</strong> polynomials<br />

converging to f compactly on D<br />

always holds for a simply connected domain. The pro<strong>of</strong> relies on Runge’s Approximation<br />

Theorem.<br />

There is also an equivalent condition for simple connectedness involving the extended<br />

complex plane C∞:<br />

(xiii) C∞ \D is connected.

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