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On Exceptional Quotient Singularities Geometry & Topology 15 (2011) 1843–1882 1843 On exceptional quotient singularities IVAN CHELTSOV CONSTANTIN SHRAMOV We study exceptional quotient singularities. In particular, we prove an exceptionality criterion in terms of the ˛–invariant of Tian, and utilize it to classify four-dimensional and five-dimensional exceptional quotient singularities. We assume that all varieties are projective, normal, and defined over C. 1 Introduction Let X be a smooth Fano variety (see Iskovskikh and Prokhorov [19]) of dimension n, and let g gi| be a Kähler metric with a Kähler form D N p 1 X ! gi| dzi dzj c1.X /: D 2 N ^ x 2 Definition 1.1 The metric g is a Kähler–Einstein metric if Ric.!/ ! , where Ric.!/ D is a Ricci curvature of the metric g. Let G Aut.X / be a compact subgroup. Suppose that g is G –invariant. x x Definition 1.2 Let P .X; g/ be the set of C 2 –smooth G –invariant functions ' such G x that x p 1 ! @@' > 0 C 2 x and sup ' 0. Then the G –invariant ˛–invariant of the variety X is the number X D x ˇ Z ˇ ' n 6 ˛G.X / sup Q ˇ C R such that e ! C for any ' PG.X; g/ : x D 2 9 2 X 2 x The number ˛G.X / was introduced by Tian [42] and Tian and Yau [44] and now it is called the ˛–invariantx of Tian. Published: 14 October 2011 DOI: 10.2140/gt.2011.15.1843 1844 Ivan Cheltsov and Constantin Shramov Theorem 1.3 [42] The Fano variety X admits a G –invariant Kähler–Einstein met- x ric if ˛G.X / > n=.n 1/. x C The normalized Kähler–Ricci flow on the smooth Fano X is defined by the equation 8 @!.t/ < Ric.!.t// !.t/; .1:4/ @t D C : !.0/ !; D where !.t/ is a Kähler form such that !.t/ c1.X /, and t R>0 . It follows from 2 2 Cao [8] that the solution !.t/ to (1.4) exists for every t > 0. Theorem 1.5 (Tian–Zhu [45]) If X admits a Kähler–Einstein metric with a Kähler form !KE , then any solution to (1.4) converges to !KE in the sense of Cheeger–Gromov. The normalized Kähler–Ricci iteration on the smooth Fano variety X is defined by the equation ( !i 1 Ric.!i/; .1:6/ D !0 !; D where !i is a Kähler form such that !i c1.X /. It follows from Yau [46] that 2 the solution !i to (1.6) exists for every i > 1. Theorem 1.7 (Rubinstein [35]) If ˛G.X / > 1 then X admits a G –invariant Kähler– x x Einstein metric with a Kähler form !KE and any solution to (1.6) converges to !KE in C 1.X /–topology. Smooth Fano varieties that satisfy all hypotheses of Theorem 1.7 do exist. Example 1.8 Let X be a smooth del Pezzo surface such that K2 5. Then X is X D unique and Aut.X / S5 . Moreover, one can show that ˛G.X / 2 in the case when Š x D G S5 or G A5 (see Cheltsov [9, Example 1.11] and Cheltsov and Shramov [11, x Š x Š Theorem A.3]). Suppose now that X P n (the simplest possible case). Then the Fubini–Study metric D on P n is Kähler–Einstein. Moreover, if G is the maximal compact subgroup of x Aut.P n/, then the only G –invariant metric on P n is the Fubini–Study metric and we n x have ˛G.P / by Definition 1.2. In particular, the solution to (1.6) is trivial x D C1 (and constant) in the latter case, since the initial metric g must be the Fubini–Study metric. On the other hand, the convergence of any solution to (1.6) is not clear in the case when G is a finite group. So, Yanir Rubinstein asked the following question in x the spring of 2009. Geometry & Topology, Volume 15 (2011) On exceptional quotient singularities 1845 n n Question 1.9 Is there a finite subgroup G Aut.P / such that ˛G.P / > 1? x x This paper is inspired by Question 1.9. In particular, we will show that the answer to Question 1.9 is positive in the case when n 6 4, which follows from [11, Theorem A.3] and Theorems 4.1, 4.2, 4.13, 5.6 and 3.21. It came as a surprise that Question 1.9 is strongly related to the notion of exceptional singularity that was introduced by Vyacheslav Shokurov in [39]. Let us recall this notion. Let .V O/ be a germ of Kawamata log terminal singularity (see Kollár [23, 3 Definition 3.5]). Definition 1.10 [39, Definition 1.5] The singularity .V O/ is said to be exceptional 3 if for every effective Q–divisor DV on the variety V such that .V; DV / is log canonical (see [23, Definition 3.5]) and for every resolution of singularities U V there W ! exists at most one –exceptional divisor E U such that a.V; DV ; E/ 1, where D the rational number a.V; DV ; E/ can be defined through the equivalence X KU DU Q .KV DV / a.V; DV ; E/E; C C C where the sum is taken over all f –exceptional divisors, and DU is the proper transform of the divisor DV on the variety U . One can show that exceptional Kawamata log terminal singularities are straightforward generalizations of the Du Val singularities of type E6 , E7 and E8 (cf Theorem 4.1), which partially justifies the word “exceptional” in Definition 1.10. Remark 1.11 One can easily check (for example, by applying Theorem 3.11) that the singularity .V O/ is not exceptional if V is smooth and dim.V / > 2. 3 It follows from Shokurov [38], Ishii and Prokhorov [18] and Markushevich and Prokhorov [27] that exceptional Kawamata log terminal singularities do exist in dimen- sions 2 and 3. The existence in dimension 4 follows from Johnson and Kollár [20] and Prokhorov [31, Theorem 4.9]. Actually, exceptional Kawamata log terminal singulari- ties exist in every dimension (see Example 3.13). We will see later (cf Theorem 1.14, Remark 1.16, Theorem 1.17 and Conjecture 1.23) that Question 1.9 is almost equivalent to the following Question 1.12 Are there exceptional quotient singularities of dimension n 1? C Geometry & Topology, Volume 15 (2011) 1846 Ivan Cheltsov and Constantin Shramov Recall that quotient singularities are always Kawamata log terminal by [23, Proposi- tion 3.16]. So Question 1.12 fits well to Definition 1.10. Moreover, it follows from Shokurov [39] and Markushevich and Prokhorov [27] that the answer to Question 1.12 is positive for n 1 and n 2, respectively. The purpose of this paper is to study D D exceptional quotient singularities and, in particular, to give positive answers to Ques- tions 1.9 and 1.12 for every n 6 4. In a subsequent paper we will show that the answers to Questions 1.9 and 1.12 are still positive for n 5 and are surprisingly negative for D n 6 (see [10]). So it is hard to predict what would be the answer to Question 1.9 in D general. However, we still believe in the following: Conjecture 1.13 For every N Z>0 there exist exceptional quotient singularities of 2 dimension greater than N . Let G be a finite subgroup in GLn 1.C/, where n > 1. Denote by Z.G/ the center and C n by ŒG; G the commutator of group G . Let GLn 1.C/ Aut.P / PGLn 1.C/ W C ! Š C be the natural projection. Put G .G/ and put x D ˇ n n ˇ the log pair .P ; D/ has log canonical singularities lct.P ; G/ sup Q ˇ : x D 2 ˇ for every G–invariant effective Q–divisor D Q KP n x n n Theorem 1.14 (See eg [11, Theorem A.3].) One has lct.P ; G/ ˛G.P /. x D x The number lct.P n; G/ is usually called G –equivariant global log canonical threshold x x of P n . Despite the fact that lct.P n; G/ ˛ .P n/, we still prefer to work with the x G n D x n number lct.P ; G/ throughout this paper, because it is easier to handle than ˛G.P /. x x For example, it follows immediately from Definition 3.1 that lct.P n; G/ 6 d=.n 1/ x C if the group G has a semi-invariant of degree d (a semi-invariant of the group G is a polynomial whose zeroes define a G –invariant hypersurface in P n ). x Remark 1.15 A semi-invariant of the group G is its invariant if Z.G/ ŒG; G and G Â x is a nonabelian simple group. Recall that an element g G is called a reflection (or sometimes a quasireflection) 2 if there is a hyperplane in P n that is pointwise fixed by .g/ (cf Springer [40, Sec- tion 4.1]). Remark 1.16 Let R G be a subgroup generated by all reflections. Then the quotient n 1 Â n 1 C C =R is isomorphic to C C (see Shephard and Todd [37] and Springer [40, Theorem 4.2.5]). Moreover, the subgroup R G is normal, and the singularity n 1 n 1 Â C C =G is isomorphic to the singularity C C =.G=R/.
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