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The construction of ALE spaces as hyper-Kähler quotients. (English) Zbl 0671.53045
Asymptotically locally Euclidean 4-manifolds were introduced over ten years ago by relativists as a gravitational analogue of the instantons of gauge theory. Such a manifold is a Riemannian manifold with the asymptotic geometry of the quotient of $$R^ 4$$ by a finite group $$\Gamma$$. The positive action theorem showed that there were no ALE solutions to Einstein’s equations with $$\Gamma =0$$ (the most natural analogue of instantons) other than flat space, but G. W. Gibbons and S. W. Hawking [Phys. Lett. B 78, 430–432 (1978)] produced self- dual examples where $$\Gamma$$ is cyclic which were geometrically non- trivial.
In this very beautiful paper, the author constructs self-dual solutions for all finite subgroups of $$\mathrm{SU}(2)\subset \mathrm{SO}(4)$$. As is well-known, these groups correspond not only to regular solids in $$R^ 3$$, but to rational double points of algebraic surfaces and Dynkin diagrams of type A, D, E. These features all play a role in the author’s construction. The metrics are produced by the hyper-Kähler quotient construction [N. J. Hitchin, A. Karlhede, U. Lindström and M. Roček Commun. Math. Phys. 108, 535–589 (1987; Zbl 0612.53043)] which starts with an action of a Lie group on a quaternionic vector space. Here the vector space and group are canonically constructed from the regular representation of the finite group $$\Gamma$$ and the complex 2-dimensional representation of $$\Gamma$$ $$\subset SU(2)$$. In working out the spaces involved, the McKay correspondence is used. This procedure produces 4- dimensional hyper-Kähler manifolds.
The next step the author takes is to identify these with the minimal resolutions of the corresponding quotient singularities $$C^ 2/\Gamma$$, a process which involves the different complex structures which a hyper- Kähler metric possesses. Finally, the ALE property is established. One of the most attractive features of the construction is the explicit way in which it produces the well-known simultaneous resolution property of rational double points in a natural (differential-) geometric context, studied earlier by E. Brieskorn [Math. Ann. 166, 76-102 (1966; Zbl 0145.094)].
Reviewer: N.Hitchin

MSC:
 53C55 Global differential geometry of Hermitian and Kählerian manifolds 53C25 Special Riemannian manifolds (Einstein, Sasakian, etc.) 14J17 Singularities of surfaces or higher-dimensional varieties
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