Comparing fundamental groups of a complex orbifolds and their resolutions.

Question

Let $X$ be a complex manifold with quotient singularities, and let $\tilde X$ be its resolution (that exists, for example, by Hironaka). Then I am pretty sure that $\pi_1(X)\cong \pi_1(\tilde X)$.

• Question. Is there a reference for such a statement? At least in dimension 3?

The reason why this should be true is that it should be possible to find such a resolution of $X$ that the preimage of each point in $\tilde X$ is simply connected ($\mathbb CP^n/G$ is simply-connected for finite $G$). Then everything follows from a standard topological lemma. I guess this statement should be true as well if $X$ is a complex analytic variety with arbitrary Kawamata log terminal singularities (because Fano manifolds are simply connected). I would be grateful for a reference for any kind of such statement.

ADDED. As Francesco says in his answer, this statement is classical for surfaces. I would like to have a reference what would cover at least the case of $3$-folds with non-isolated quotient singularities (say with abelian stabilisers).

UPD. The question is now 100% settled by the reference provided by the answer of Benoit.

Answer 1

A reference is Theorem 7.8 of the article by Kollar: "Shafarevich maps and plurigenera of algebraic varieties", Invent. Math. 113. This proves the equality of fundamental groups for quotient singualrities in all dimensions and also for algebraic fundamental groups in the case of klt singularities.

Answer 2

The proof of simple connectedness of a resolution of quotient singularities is in this paper of mine http://arxiv.org/abs/math/9903175 Theorem 4.1 (published in Asian J. Math. 4, 2000, no. 3, 553-563), but I guess it was known to Bogomolov long ago (and maybe even published in some of his papers)

Answer 3

Dear Dmitri, the result you hope is also true (ie when X has only klt singularities, its fundamental group is isomorphic to the one of any of its desingularization). This theorem is due to Takayama (Local simple connectedness of resolution of log-terminal singularities, International journal of Math 2003).

Bests, Benoît

Answer 4

Your statement is surely true in the following two cases:

1. $\dim X=2$. In this case something stronger is true; in fact, if $X$ is a surface with only rational singularities and $\tilde{X}$ is a resolution of singularities, then $\pi_1(\tilde{X})=\pi_1(X)$. This because the resolution of a rational $2$-dimensional singularity is alwais given by a bunch of rational curves, in particular the exceptional locus is simply connected, so you can conclude by Seifert-Van Kampen theorem. Thi argument is used for instance in the paper of R. Barlow

"A symply connected surface of general type with $p_g=0$", Invent. Math.79.

1. $\dim X$ is arbitrary and all singularities of $X$ are isolated cyclic quotient singularities. In fact, by a result of Fujiki, an isolated cyclic quotient singularity can always be resolved by a normal crossing divisor whose components are smooth rational varieties, so you can apply again Seifert-Van Kampen theorem.

This argument is used in the paper of Bauer, Catanese, Pignatelli, Grunewald

Quotients of products of curves, new surfaces with $p_g=0$ and their fundamental groups

in order to compute the fundamental group of a quotient of the form $(C_1 \times C_2 \times \cdots \times C_n)/G$, where the $C_i$ are smooth curves (see in particular Remark 2.4).

I do not know whether the result is true in full generality. In fact, given a finite group $G$ acting on a simply connected manifold $V$, by a result of Armstrong (Proc. Amer. Math. Soc. 84) one has

$\pi_1(V/G)=G/E$,

where $E$ is the subgroup of elements having fixed points.

In your case $V=\mathbb{C}^n$, and if $\mathbb{C}^n/G$ is simply connected, i.e. if $G=E$, then you can apply Seifert-Van Kampen again. Therefore it seems to me that your question is related to the following one:

Does any cyclic group $\langle g \rangle$ acting on $\mathbb{C}^n$ have a fixed point?

If $g$ is a polynomial automorphism then the answer is yes, but in general this is an open problem for $n \geq 3$ (for $n=2$ it is true since any finite group acting on $\mathbb{C}^2$ is linearizable by a result of Suzuki).

You can find more on this topic (and several related references) in the paper by Kraft and Schwarz "Finite automorphisms of affine $n$-spaces".