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Tim Perutz
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An illustrative example: the moduli space $M$ of regular pentagons with edges of unit length. This embeds as an open, dense subset of a compact complex surface $\bar{M}$ with a canonical Kaehler form. This surface, a 4-fold blow-up of $\mathbb{CP}^2$, is not Calabi-Yau, however.

The compactified regular pentagon space $\bar{M}$ is the space of 5-tuples of unit vectors in $\mathbb{R}^3$ with centre of mass zero, modulo the diagonal action of $SO(3)$. Since we remember the order of the factors, typical points represent regular pentagons with labelled vertices. There are also points which represent an equilateral triangle together with a pair of antipodal points, and these non-pentagon points form ten 2-spheres in $\bar{M}$.

$\bar{M}$ has a natural symplectic structure, for which the ten 2-spheres are Lagrangian. Up to scale, there's a unique area-form on $S^2$ invariant under $SO(3)$, and the moment map $S^2\to \mathfrak{so}(3)^\ast \cong \mathbb{R}^3$ for this $SO(3)$-action is just the inclusion of $S^2$ into $\mathbb{R}^3$. The product $(S^2)^5$ carries a product symplectic form, again $SO(3)$-invariant, with moment map $\mu(x_1,\dots,x_5)=x_1+\dots + x_5\in \mathbb{R}^3$. The symplectic quotient $\mu^{-1}(0)/SO(3)$ is just $\bar{M}$.

The action of $SO(3)=PU(2)$ respects the complex structure, and $\bar{M}$ inherits a complex structure (by Kaehler reduction). It turns out to be isomorphic as a complex surface to a blow up of $\mathbb{CP}^2$ at four special points. See Seidel's Lectures on 4-dimensional Dehn twists, ex. 1.10. There's a natural action of the icosahedral group.

If one wanted pentagons defined by some other linear equation, say $a_1x_1+\dots + a_5x_5=0$, one would give the $S^2$-factors areas $a_i$.

One can also interpret $\bar{M}$ as an algebro-geometric (GIT) quotient of $(\mathbb{CP}^1)^5$ by $PSL_2(\mathbb{C})$. The quotient happens to be the Deligne-Mumford compactification $\bar{M}_{0,5}(\mathbb{C})$ of configurations of five points on $\mathbb{CP}^1$. The real points $\bar{M}_{0,5}(\mathbb{R})$ (the fixed points of an anti-holomorphic involution of $\bar{M}$) are also interesting: their connected components are polyhedral and are copies of the 2-dimensional Stasheff associahedron (a.k.a. pentagon).

Reference: F. Kirwan, "Cohomology of quotients in symplectic and algebraic geometry".

Tim Perutz
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