ScalingStacks

Proof. [05AL]

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Proof.

Let 𝔛an=βˆ‘jmj​Xj\mathfrak{X}^{\textup{an}}=\sum_{j}m_{j}X_{j} be the decomposition into prime cycles. It is enough to prove the claim for the closures XΒ―j\overline{X}_{j} of XjX_{j} in 𝔛\mathfrak{X}. We may hence assume that 𝔛an\mathfrak{X}^{\textup{an}} is irreducible and reduced. Furthermore by passing to a dominating model as in 4, we may assume that the special fibre 𝔛~\tilde{\mathfrak{X}} of 𝔛\mathfrak{X} is reduced. As 𝔛an\mathfrak{X}^{\textup{an}} has no boundary, every irreducible component of 𝔛~\tilde{\mathfrak{X}} is proper by Corollary A.4 and hence using commutativity of the intersection product ([Gub98, Theorem 5.9]) we obtain

βˆ«π”›anf0​c1​(𝔏1)βˆ§β€¦βˆ§c1​(𝔏n)\displaystyle\int_{\mathfrak{X}^{\textup{an}}}f_{0}\;c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) =βˆ‘Y∈irr⁑(𝔛~)f0​(ΞΆY)β‹…deg𝔏1,…,𝔏n⁑(Y)\displaystyle=\sum_{\begin{subarray}{c}Y\in\irr(\tilde{\mathfrak{X}})\end{subarray}}f_{0}(\zeta_{Y})\cdot\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)
=deg𝔏1,…,𝔏n⁑(cyc⁑(div𝔏0⁑(1)))\displaystyle=\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}\left(\cyc(\Div_{\mathfrak{L}_{0}}(1))\right)
=deg𝔏0,𝔏2,…,𝔏n⁑(cyc⁑(div𝔏1⁑(1)))\displaystyle=\Deg_{\mathfrak{L}_{0},\mathfrak{L}_{2},...,\mathfrak{L}_{n}}\left(\cyc(\Div_{\mathfrak{L}_{1}}(1))\right)
=βˆ‘Y∈irr⁑(𝔛~)f1​(ΞΆY)β‹…deg𝔏0,𝔏2,…,𝔏n⁑(Y)\displaystyle=\sum_{\begin{subarray}{c}Y\in\irr(\tilde{\mathfrak{X}})\end{subarray}}f_{1}(\zeta_{Y})\cdot\Deg_{\mathfrak{L}_{0},\mathfrak{L}_{2},...,\mathfrak{L}_{n}}(Y)
=βˆ«π”›anf1​c1​(𝔏0)∧c1​(𝔏2)βˆ§β€¦βˆ§c1​(𝔏n).\displaystyle=\int_{\mathfrak{X}^{\textup{an}}}f_{1}\;c_{1}(\mathfrak{L}_{0})\wedge c_{1}(\mathfrak{L}_{2})\wedge...\wedge c_{1}(\mathfrak{L}_{n}).

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