ScalingStacks

3.2.1. ALF spaces of cyclic type [02GZ]

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3.2.1. ALF spaces of cyclic type

We saw that gravitational instantons of type AkA_{k} can be constructed from Dirac monopoles on ℝ3\mathbb{R}^{3} with k+1k+1 singularities via the Gibbons–Hawking ansatz. These are usually called multi-Taub–NUT metrics. The case k=0k=0 is the Taub–NUT metric on ℝ4\mathbb{R}^{4} and k=−1k=-1 is ℝ3×𝕊1\mathbb{R}^{3}\times\mathbb{S}^{1} with its flat metric. Minerbe [35, Theorem 0.2] has shown that every ALF space of cyclic type must be isometric to a multi-Taub–NUT metric.

From their explicit description one can easily compute basic information about cyclic ALF spaces: the fundamental group π1​(M)\pi_{1}(M), the second Betti number b2​(M)b_{2}(M), the Euler characteristic and the dimension of the moduli space ℳ\mathcal{M} of AkA_{k} metrics:

kk π1​(M)\pi_{1}(M) b2​(M)b_{2}(M) χ⁡(M)\chi(M) dim​(ℳ)\text{dim}(\mathcal{M})
−1-1 ℤ\mathbb{Z} 00 00 00
k>−1k>-1 11 kk k+1k+1 3​k3k

Here we assume that the asymptotic length of the circle fibre is normalised to be 11 so that dim​(ℳ)\text{dim}(\mathcal{M}) does not include rescalings.

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