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5. Moduli of Kähler K3 surfaces [04YS]

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5. Moduli of Kähler K3 surfaces

It is known (cf., [Tod80], [Looi81], [KT87]) that the moduli space of all Einstein metrics on a Kähler K3 surfaces (including orbifold-metrics) has again a structure of the locally Riemannian symmetric space:

O⁡(ΛK3)\S​O0​(3,19)/(S​O​(3)×S​O​(19)),O(\Lambda_{\rm K3})\backslash SO_{0}(3,19)/(SO(3)\times SO(19)),

which we denote by ℳK3\mathcal{M}_{\rm K3}. An enriched version encoding also complex structures of the K3 surfaces is

ℝ>0×(O⁡(ΛK3)\S​O0​(3,19)/(S​O​(2)×S​O​(19))).\mathbb{R}_{>0}\times(O(\Lambda_{\rm K3})\backslash SO_{0}(3,19)/(SO(2)\times SO(19))).

Roughly speaking, this is a union of Kähler cones of ADE K3 surfaces with marking of the minimal resolutions.

Thus we can again compare a Satake compactification of ℳK3\mathcal{M}_{\rm K3} with the Gromov-Hausdorff compactification. Inside the Satake compactification for the adjoint representation, we consider an open locus (a partial compactification of ℳK3\mathcal{M}_{\rm K3}) ℳK3⊔ℳK3​(a),\mathcal{M}_{\rm K3}\sqcup\mathcal{M}_{\rm K3}(a), where ℳK3​(a)\mathcal{M}_{\rm K3}(a) denotes the 3636-dimensional boundary stratum corresponding to an isotropic rational line l=ℚ​el=\mathbb{Q}e in ΛK3⊗ℚ\Lambda_{\rm K3}\otimes\mathbb{Q}, with primitive integral generator ee, which are unique up to O⁡(ΛK3)O(\Lambda_{\rm K3}). Then for each point p=⟨e,v1,v2⟩p=\langle e,v_{1},v_{2}\rangle in strata ℳK3​(a)\mathcal{M}_{\rm K3}(a), we consider the marked (possibly ADE) K3 surface XpX_{p} with period ⟨v1,v2⟩\langle v_{1},v_{2}\rangle. Then it is known that there is an elliptic K3 surface structure on XpX_{p} with the fiber class ee. Then we define Φ⁡(p)\Phi(p) as its base biholomorphic to ℙ1\mathbb{P}^{1} with the McLean metric, which only depends on ⟨v1,v2⟩\langle v_{1},v_{2}\rangle. Similarly to the projective case Theorem 4.4, [OO] proves that for non-algebraic situation:

Theorem 5.1.

The map

Φ:ℳK3⊔ℳK3​(a)→𝐶𝑀𝑒𝑡1\Phi\colon\mathcal{M}_{\rm K3}\sqcup\mathcal{M}_{\rm K3}(a)\to{\it CMet}_{1}

given above is continuous. Here, we put the Gromov-Hausdorff topology for the right hand side.

In [OO], we further explicitly define an extension to the whole Satake compactification Φ:ℳK3¯Sat→𝐶𝑀𝑒𝑡1\Phi\colon\overline{\mathcal{M}_{\rm K3}}^{\rm Sat}\to{\it CMet}_{1}, and conjecture that this is still continuous with respect to the Gromov-Hausdorff topology. For the boundary strata other than ℳK3​(a)\mathcal{M}_{\rm K3}(a), we assign flat tori ℝi/ℤi​(i=1,2,3)\mathbb{R}^{i}/\mathbb{Z}^{i}\ (i=1,2,3) modulo (−1)(-1)-multiplication. We show that Φ\Phi restricted to the closure of the locus which parametrizes ℝ4/ℤ4\mathbb{R}^{4}/\mathbb{Z}^{4} modulo ±1\pm 1, that includes those boundary strata, is continuous. Furthermore, we also prove the restriction of Φ\Phi to the closure of ℳK3​(a)\mathcal{M}_{\rm K3}(a) is continuous by using Weierstrass models.

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