ScalingStacks

Example 5.4 . [00AT]

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Example 5.4.

For m=1m=1, consider a 00-dimensional face ΔJ\Delta_{J} of S​k​(X)Sk(X), such that there are two 11-dimensional faces of Δ𝒳\Delta_{\mathcal{X}} containing ΔJ\Delta_{J}, and they both lie on S​k​(X)Sk(X). A very simple geometric situation is when EJE_{J} is the smooth total space of a possibly singular ℙ1\mathbb{P}^{1}-fibration over an (n−1)(n-1)-dim smooth variety DD, and the two 1-dimensional faces correspond to two disjoint sections EJ′,1,EJ′,2E_{J^{\prime},1},E_{J^{\prime},2} of the ℙ1\mathbb{P}^{1}-fibration, so EJ′,1≃EJ′,2≃DE_{J^{\prime},1}\simeq E_{J^{\prime},2}\simeq D. A natural way to make 𝒟J​(x,‖⋅‖C​Y)n⋅EJ=0\mathcal{D}_{J}(x,\left\lVert\cdot\right\rVert_{CY})^{n}\cdot E_{J}=0 and 𝒟J​(x,‖⋅‖C​Y)\mathcal{D}_{J}(x,\left\lVert\cdot\right\rVert_{CY}) nef for x∈ΔJx\in\Delta_{J}, is to ask 𝒟J​(x,‖⋅‖C​Y)\mathcal{D}_{J}(x,\left\lVert\cdot\right\rVert_{CY}) to be the pullback of a nef class on the base DD. We regard this nef class as the limiting element of 𝒟J​(y,‖⋅‖C​Y)∈H1,1​(EJ′,i)\mathcal{D}_{J}(y,\left\lVert\cdot\right\rVert_{CY})\in H^{1,1}(E_{J^{\prime},i}) as yy approaches xx from either of the 11-dimensional faces. Then the matching condition is naturally seen as the continuity of 𝒟J​(y,‖⋅‖C​Y)\mathcal{D}_{J}(y,\left\lVert\cdot\right\rVert_{CY}) across the 00-dimensional face. This example may be relevant for the Ooguri-Vafa type neck region in [25][42] (cf. [42, section 7.1]). The possibility for the ℙ1\mathbb{P}^{1}-fibration to develop nodal fibres is related to the monopole bubbling phenemenon in these papers.

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