Nonexistence of conformal map from the punctured unit disk to the annulus

complex-analysissolution-verification

Prove that there is no one-to-one conformal map ($=:f$) from the punctured unit disk $\{z:0<|z|<1\}$ onto the annulus $\{z:1<|z|<2\}$.

Proof. Any holomorphic function from the punctured unit disk to the annulus is bounded near $0$ so can be extended to a function that is holomorphic at $0$. In particular it has a square root as it's a nonzero holomorphic function on a simply connected region. However there is a holomorphic function from the annulus to itself without a square root (for example the identity function). So the punctured unit disk cannot be conformal to the annulus.

Let $\tilde{f}$ be the extension of $f$ on the unit disk $\Bbb D$. I understand the existence of square root $g$ i.e., $g^2 = \tilde{f}$. But I can't see why this implies the existence of a holomorphic function from the annulus to itself (so the contradiction makes sense). Can you explain this?

Best Answer

Note that if $g:\mathbb D^* \to \mathbb C$ is holomorphic and both bounded inferior and superior ($0<m<|g(z)|<M$), then $g$ extends holomorphically to a nonvanishing function on the unit disc, so has a holomorphic square root $h^2=g$

Now if $f :\mathbb D^* \to A$ is a holomorphic isomorphism where $A$ is the annulus $1<|z|<2$ (though any proper annulus will do), then if $I:A \to A$ is the identity function, $I\circ f$ is holomorphic and both bounded inferior and superior on the punctured disc, so has a square root $h^2=I\circ f$ which implies that $I(z)=h^2(f^{-1}(z))=(h(f^{-1}(z)))^2$ so getting the required contradiction as we know that $I$ doesn't have a square root ($k^2=I$ implies $2k'/k=I'/I$ and integrating $0=\frac{1}{2\pi i}(2k'/k-I'/I)$ on the circle of radius $3/2$ we get $0=1$)

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