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lines changed Original file line number Diff line number Diff line change 1- \subsubsection {Hirzebruch's smooth compactification }\label {H-compact }
1+ \subsubsection {Hartman's less than smooth compactification }\label {H-compact }
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5- We let $ X'$ be the Baily-Borel compactification of $ X$ , which is
6- obtained by collapsing in $ \overline {X}$ each boundary component
7- $ e'(P)$ to a single point or topologically by taking a cone on each
8- component of the Borel-Serre boundary. It is well known that $ X'$
9- is a projective algebraic variety. We let $ \tilde {X}$ be Hirzebruch's
10- smooth resolution of the cusp singularities and $ \pi :\tilde {X} \to
11- X'$ be the natural map collapsing the compactifying divisors for
12- each cusp. We let $ j:X \hookrightarrow \tilde {X}$ be the natural
13- embedding. Note that the Borel-Serre boundary separates $ \tilde {X}$
14- into two pieces, the (connected) inside $ X^{in}$ , which is isomorphic
15- to $ X$ and the (disconnected) outside $ X^{out}$ , which for each
16- cusp is a neighborhood of the compactifying divisors. Note that we
17- can view $ e'(P)$ as lying in both $ X^{in}$ and $ X^{out}$ since the
18- intersection $ X^{in} \cap X^{out}$ is equal to $ \coprod _{\underline {P}}
19- e({P})$ .
5+ We let $ X'$ be the Baily-Borel compactification of $ X$ , then we ignore it forever. I'm pretty sure they just made this all up.
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