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Brian Rushton
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Horrible sets and blowups in Hubbard's Teichmuller theory

I've had this question for some time. In Hubbard's Teichmuller theory book, on page 9, he describes an ugly complex 2-manifold that is not second countable. He constructs it by taking $\mathbb{C}^2$ and blowing up every point along the axis $\mathbb{C} \times 0$. More specifically, he considers all blowups of finitely many points along this axis, and takes their inverse limit under the natural projection maps from one blowup to another with a strictly smaller subset of blowup points. There is a natural map $p$ from the blown-up space to $\mathbb{C}^2$.

He denotes $p^{-1}(\mathbb{C} \times 0)$ by $Y$ and claims that $Y$ consists of one copy of projective space for each point of the complex line plus a 'horrible set'. Here's my question:

Where does the horrible set come from?

Because every point in the inverse limit is given by $\mathop{\Pi} \limits_{\alpha \in J} a_\alpha \in \mathop{\Pi} \limits_{\alpha \in J} X_\alpha$ with $p_{\alpha\beta}(a_\alpha)=a_\beta$ whenever $\alpha<\beta$ (here the $X_\alpha$ are all the blowups with partial order given by inclusion of the blow up points and the p maps are the natural projections). So, any point in the inverse limit has a coordinate $a_0$ in the non-blown up space $X_0=\mathbb{C}^2$ (the minimal element in the ordering), so it must live in the blowup $a_0$ of that point, which is a copy of projective space, and not in the horrible set. So where does the horrible set come from?

Brian Rushton
  • 3.4k
  • 8
  • 38
  • 63