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This is a silly question, but I can't resolve this:

$Y(1)$ is defined to be $\mathbb{H}/PSL_2(\mathbb{Z})$. So it seems that its universal cover should be $\mathbb{H}$.

On the other hand $Y(1)$ is isomorphic to $\mathbb{A}^1_{\mathbb{C}}$, and therefore is its own universal cover.

What am I missing?

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    Why do you think the fact that $Y(1)$ is a quotient of the upper-half plane implies that its universal cover _is_ the upper-half plane?2011-10-29
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    I can't think of points where it is branched, which would make it a covering map...2011-10-29
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    Dear Nicole, There is branching at the points corresponding to $j=0$ and $j=1728$. Not coincidentally, these are the $j$-invariants corresponding to elliptic curves with extra automorphisms. Regards,2011-11-25

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To follow up on Bruno Joyal's comment, $PSL(2,\mathbb Z)$ does not act freely on $\mathbb H$. If you think of this group as being generated by $z\mapsto -1/z$ and $z\mapsto z+1$, then the complex number $i$ is fixed by the first map, so it has a nontrivial stabilizer.