ScalingStacks

Example 2.7 (Generic singular fibration) . [04HR]

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Example 2.7 (Generic singular fibration).

This example is the model for the fibration over a neighborhood of an edge point of Δ\Delta –in [7] this is called (2,2)(2,2) fibration. Let B=D×(0,1)B=D\times(0,1), where D⊂ℂD\subset\mathbb{C} is the unit disc, and let Y=T2×BY=T^{2}\times B. Define Σ⊂Y\Sigma\subset Y to be the cylinder sitting above {0}×(0,1)⊂B\{0\}\times(0,1)\subset B defined as follows. Let e1,e3e_{1},e_{3} be a basis of H1​(T2,ℤ)H_{1}(T^{2},\mathbb{Z}). Let S1⊂T2S^{1}\subset T^{2} be a circle representing the homology class e3e_{3}. Define Σ=S1×{0}×(0,1)\Sigma=S^{1}\times\{0\}\times(0,1). Now let π′:X′→Y′:=Y−Σ\pi^{\prime}:X^{\prime}\rightarrow Y^{\prime}:=Y-\Sigma be an S1S^{1}-bundle with Chern class c1=1c_{1}=1. Then X′X^{\prime} compactifies to a manifold X=X′∪ΣX=X^{\prime}\cup\Sigma and there is a proper map π:X→Y\pi:X\rightarrow Y extending π′\pi^{\prime}. We can now define f=P∘π:X→Bf=P\circ\pi:X\rightarrow B where P:Y→BP:Y\rightarrow B is the projection. Then it is clear ff is a T3T^{3} fibration with singular fibres homeomorphic to I1×S1I_{1}\times S^{1} lying over Δ:={0}×(0,1)\Delta:=\{0\}\times(0,1). If e2e_{2} is an orbit of π\pi, one can take e1,e2,e3e_{1},e_{2},e_{3} as a basis of H1​(Xb,ℤ)H_{1}(X_{b},\mathbb{Z}), where XbX_{b} is a regular fibre. In this basis, e2e_{2} and e3e_{3} are monodromy invariant and a generator of the monodromy group of ff about Δ\Delta is represented in this basis by

T=(100110001).T=\left(\begin{array}[]{ccc}1&0&0\\ 1&1&0\\ 0&0&1\end{array}\right). (2)

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