Algebraic Montgomery-Yang Problem: the noncyclic case

Details Algebraic Montgomery-Yang Problem: the noncyclic case Algebraic Montgomery-Yang Problem: the noncyclic case

2009-04-20

arxiv.org/abs/0904.2975v5

Mathematics

Algebraic Geometry

26 pages. A statement in Theorem 1.5(4) is corrected, hence the corresponding statements in Proposition 3.5 and Example 3.6 ar

Scientific paper

Montgomery-Yang problem predicts that every pseudofree differentiable circle action on the 5-dimensional sphere ${\mathbb S}^5$ has at most 3 non-free orbits. Using a certain one-to-one correspondence, Koll\'ar formulated the algebraic version of the Montgomery-Yang problem: every projective surface $S$ with quotient singularities such that $b_2(S) = 1$ has at most 3 singular points if its smooth locus $S^0$ is simply-connected. In this paper, we prove the conjecture under the assumption that $S$ has at least one noncyclic singularity. In the course of the proof, we classify projective surfaces $S$ with quotient singularities such that (i) $b_2(S) = 1$, (ii) $H_1(S^0, \mathbb{Z}) = 0$, and (iii) $S$ has 4 or more singular points, not all cyclic, and prove that all such surfaces have $\pi_1(S^0)\cong \mathfrak{A}_5$, the icosahedral group.

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