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Let $X$ be a compact connected Riemann surface.

Let $\pi:X\to \mathbf{P}^1$ be a gonal morphism, i.e., a morphism of minimal degree.

Can $\pi$ have non-trivial automorphisms? (An automorphism of $\pi$ is an automorphism of $\sigma:X\to X$ of $X$ such that $\pi\circ \sigma = \pi$.)

Is this true if $\pi$ is hyperelliptic, i.e., if $\deg \pi = 2$ and $g\neq 1$?

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For the definition of hyperelliptic, I think you mean deg$\pi=2$ and $g\gt 1$ –  Georges Elencwajg Feb 24 '12 at 21:26
    
You're right. I changed it. (The case $g=0$ is automatically excluded because $\mathbf{P}^1$ has gonality $0$.) –  Hoedan Feb 24 '12 at 21:34
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If $\pi$ is a Galois cover (e.g. when $X$ is hyperelliptic), then the set of the automorphisms of $\pi$ is the Galois group of the cover, thus is non-trivial if $\deg \pi >1$.

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