ScalingStacks

Example 5.3 . [02ZK]

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Example 5.3.

Consider a degenerating family of elliptic curves parameterized by tt, given by ℂ/(ℤ+ℤ​τ)\mathbb{C}/(\mathbb{Z}+\mathbb{Z}\tau) where 11 and τ=12​π​i​log⁡t\tau={1\over 2\pi i}\log t are periods of the elliptic curves. If we take tt approaching 00 along the positive real axis, then we can just view this as a family of elliptic curves 𝒳α\mathcal{X}_{\alpha} with period 11 and i​αi\alpha with α→∞\alpha\rightarrow\infty. If we take the standard Euclidean metric gg on 𝒳α\mathcal{X}_{\alpha}, then the diameter of 𝒳α\mathcal{X}_{\alpha} is unbounded. To obtain a bounded diameter, we replace gg by g/α2g/\alpha^{2}; equivalently, we can keep gg fixed on ℂ\mathbb{C} but change the periods of the elliptic curve to 1/α,i1/\alpha,i. It then becomes clear that the Gromov-Hausdorff limit of such a sequence of elliptic curves is a circle ℝ/ℤ\mathbb{R}/\mathbb{Z}.

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