
Algebra, Arithmetic, and Geometry
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J´anos Koll´ar and Michael Larsen
Summary. Let X be a complex Calabi–Yau variety, that is, a complex projective variety with canonical singularities whose canonical class is numerically trivial. Let G be a ?nite group acting on X and consider the quotient variety X/G. The aim of this paper is to determine the place of X/G in the birational classi?cation of varieties. That is, we determine the Kodaira dimension of X/G and decide when it is uniruled or rationally connected. If G acts without ?xed points, then κ(X/G) = κ(X) = 0; thus the interesting case is when G has ?xed points. We answer the above questions in terms of the action of the stabilizer subgroups near the ?xed points. We give a rough classi?cation of possible stabilizer groups which cause X/G to have Kodaira dimension −∞ or equivalently (as we show) to be uniruled. These stabilizers are closely related to unitary re?ection groups.
Key words: Calabi–Yau, uniruled, rationally connected, re?ection group
2000 Mathematics Subject Classi?cations: 14J32, 14K05, 20E99 (Primary) 14M20, 14E05, 20F55 (Secondary)
Let X be a Calabi–Yau variety over C, that is, a projective variety with canonical singularities whose canonical class is numericaly trivial. Let G be a ?nite group acting on X and consider the quotient variety X/G. The aim of this paper is to determine the place of X/G in the birational classi?cation of varieties. That is, we determine the Kodaira dimension of X/G and decide when it is uniruled or rationally connected.
If G acts without ?xed points, then κ(X/G) = κ(X) = 0; thus the interesting case is that in which G has ?xed points. We answer the above questions in terms of the action of the stabilizer subgroups near the ?xed points. The answer is especially nice if X is smooth. In the introduction we concentrate on this case. The precise general results are formulated later. Definition 1. Let V be a complex vector space and g ∈ GL(V ) an element of ?nite order. Its eigenvalues (with multiplicity) can be written as"
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