ScalingStacks

Proof. [05AJ]

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Proof.

ii) follows from symmetry and multilinearity of the intersection product ([Ful98, Proposition 2.5]). For iii) we reduce first to the case where 𝔛′\mathfrak{X}^{\prime} and 𝔛\mathfrak{X} have reduced special fibre. Let π”œβ€²\mathfrak{Y}^{\prime} respectively π”œ\mathfrak{Y} be the canonical formal models with reduced special fibre as in 4. This construction is functorial and we obtain a commutative diagram

π”œβ€²\textstyle{\mathfrak{Y}^{\prime}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}ΞΉβ€²\scriptstyle{\iota^{\prime}}Ο†β€²\scriptstyle{\varphi^{\prime}}π”œ\textstyle{\mathfrak{Y}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}ΞΉ\scriptstyle{\iota}𝔛′\textstyle{\mathfrak{X}^{\prime}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ο†\scriptstyle{\varphi}𝔛\textstyle{\mathfrak{X}}

Assuming that we know the claim for reduced special fibres we obtain

deg⁑(Ο†an)​c1​(𝔏1)βˆ§β€¦βˆ§c1​(𝔏n)\displaystyle\Deg(\varphi^{\textup{an}})c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) =deg⁑(Ο†β€²a​n)​(ΞΉan)βˆ—β€‹(c1​(ΞΉβˆ—β€‹π”1)βˆ§β€¦βˆ§c1​(ΞΉβˆ—β€‹π”n))\displaystyle=\Deg(\varphi^{\prime an})(\iota^{\textup{an}})_{\ast}(c_{1}(\iota^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\iota^{\ast}\mathfrak{L}_{n}))
=(ΞΉan)βˆ—β€‹(Ο†β€²a​n)βˆ—β€‹(c1​(Ο†β€²β£βˆ—β€‹ΞΉβˆ—β€‹π”1)βˆ§β€¦βˆ§c1​(Ο†β€²β£βˆ—β€‹ΞΉβˆ—β€‹π”n))\displaystyle=(\iota^{\textup{an}})_{\ast}(\varphi^{\prime an})_{\ast}(c_{1}(\varphi^{\prime\ast}\iota^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\prime\ast}\iota^{\ast}\mathfrak{L}_{n}))
=(Ο†an)βˆ—β€‹(ΞΉβ€²a​n)βˆ—β€‹(c1​(ΞΉβ€²β£βˆ—β€‹Ο†βˆ—β€‹π”1)βˆ§β€¦βˆ§c1​(ΞΉβ€²β£βˆ—β€‹Ο†βˆ—β€‹π”n))\displaystyle=(\varphi^{\textup{an}})_{\ast}(\iota^{\prime an})_{\ast}(c_{1}(\iota^{\prime\ast}\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\iota^{\prime\ast}\varphi^{\ast}\mathfrak{L}_{n}))
=(Ο†an)βˆ—β€‹(c1​(Ο†βˆ—β€‹π”1)βˆ§β€¦βˆ§c1​(Ο†βˆ—β€‹π”n)).\displaystyle=(\varphi^{\textup{an}})_{\ast}(c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})).

So from now on assume that 𝔛′\mathfrak{X}^{\prime} and 𝔛\mathfrak{X} have reduced special fibre. Let YY be an irreducible component of 𝔛~\tilde{\mathfrak{X}} with corresponding Shilov point ΞΆY\zeta_{Y}. Let ΞΆ1,…,ΞΆr\zeta_{1},...,\zeta_{r} be the preimages of ΞΆY\zeta_{Y} under Ο†an\varphi^{\textup{an}} with corresponding irreducible components Y1,…,YrY_{1},...,Y_{r} of 𝔛′~\tilde{\mathfrak{X}^{\prime}}. If YY is proper then clearly all the YiY_{i} are proper. If on the other hand one of the YiY_{i} is proper then YY is proper by [GW10, Proposition 12.59]. In this case we can use the projection formula to calculate:

(Ο†an)βˆ—β€‹(c1​(Ο†βˆ—β€‹π”1)βˆ§β€¦βˆ§c1​(Ο†βˆ—β€‹π”n))​(ΞΆY)\displaystyle(\varphi^{\textup{an}})_{\ast}\left(c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})\right)(\zeta_{Y}) =βˆ‘i=1rc1​(Ο†βˆ—β€‹π”1)βˆ§β€¦βˆ§c1​(Ο†βˆ—β€‹π”n)​(ΞΆi)\displaystyle=\sum_{i=1}^{r}c_{1}(\varphi^{\ast}\mathfrak{L}_{1})\wedge...\wedge c_{1}(\varphi^{\ast}\mathfrak{L}_{n})(\zeta_{i})
=βˆ‘i=1rdeg𝔏1,…,𝔏n⁑(Ο†~βˆ—β€‹Yi)\displaystyle=\sum_{i=1}^{r}\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(\tilde{\varphi}_{\ast}Y_{i})
=βˆ‘i=1rdeg𝔏1,…,𝔏n(Y)β‹…[K~(Yi):K~(Y)]\displaystyle=\sum_{i=1}^{r}\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)\cdot[\tilde{K}(Y_{i}):\tilde{K}(Y)]

As already mentioned in Definition 4.2, Ο†an\varphi^{\textup{an}} is finite outside a lower dimensional analytic subset. Hence we may apply equation (3) in the proof of [Gub98, Proposition 4.5] to see that the last term in the display equals deg⁑(Ο†an)β‹…c1​(𝔏1)βˆ§β€¦βˆ§c1​(𝔏n)​(ΞΆY)\Deg(\varphi^{\textup{an}})\cdot c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n})(\zeta_{Y}).
On the other hand, if YY is an irreducible component of 𝔛~β€²\tilde{\mathfrak{X}}^{\prime} whose image is not an irreducible component of 𝔛~\tilde{\mathfrak{X}} then its degree with respect to the line bundles Ο†βˆ—β€‹π”1,…,Ο†βˆ—β€‹π”n\varphi^{\ast}\mathfrak{L}_{1},...,\varphi^{\ast}\mathfrak{L}_{n} is 00 by the projection formula, as the image is of lower dimension. This proves iii).
For i) let 𝔛an=βˆ‘jmj​Xj\mathfrak{X}^{\textup{an}}=\sum_{j}m_{j}X_{j} be the decomposition into prime cycles. It is then enough to prove the claim for each XjX_{j} and by definition of the measure we may hence assume that 𝔛\mathfrak{X} has irreducible and reduced generic fibre and reduced special fibre. Let SS be the set of all ΞΆY\zeta_{Y} where YY is a proper irreducible component of 𝔛~\tilde{\mathfrak{X}} with deg𝔏1,…,𝔏n⁑(Y)β‰ 0\Deg_{\mathfrak{L}_{1},...,\mathfrak{L}_{n}}(Y)\neq 0. Then SS is discrete as redβˆ’1⁑(Y)\red^{-1}(Y) is an open neighbourhood of ΞΆY\zeta_{Y} which does not contain any other points of SS. Furthermore XX is the union of all redβˆ’1⁑(Y)\red^{-1}(Y) where YY runs over all irreducible components of 𝔛~\tilde{\mathfrak{X}} and as all of these sets contain at most one point of SS and by paracompactness of XX, every xβˆ‰Sx\notin S has an open neighbourhood which does not intersect SS and hence SS is closed. By definition c1​(𝔏1)βˆ§β€¦βˆ§c1​(𝔏n)c_{1}(\mathfrak{L}_{1})\wedge...\wedge c_{1}(\mathfrak{L}_{n}) is of the desired form and its support is contained in the relative interior of XX over KK by Corollary A.4. ∎

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