ScalingStacks

5.8. Residual boundaries [0173]

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5.8. Residual boundaries

The following construction plays a crucial role for the understanding of the limit measure appearing in Corollary B.

Consider a model metric ℒ{\mathcal{L}} of KXK_{X} defined on a proper dlt model 𝒳{\mathcal{X}}. Following §3.1 we explain how to associate a subklt pair (Y,BYℒ)(Y,B^{\mathcal{L}}_{Y}) to each stratum YY of 𝒳0{\mathcal{X}}_{0} corresponding to a maximal simplex in Δ⁡(ℒ)\Delta({\mathcal{L}}).

Let us first recall a few facts about adjunction. When 𝒳{\mathcal{X}} is an snc model, each stratum YY comes with a boundary BY:=∑i∉JYEi∩YB_{Y}:=\sum_{i\notin J_{Y}}E_{i}\cap Y. Here (Y,BY)(Y,B_{Y}) is log smooth, and

K𝒳/Slog|Y=K(Y,BY):=KY+BY,K^{\mathrm{log}}_{{\mathcal{X}}/S}\big|_{Y}=K_{(Y,B_{Y})}:=K_{Y}+B_{Y}, (5.6)

the identification being provided by Poincaré residues. When 𝒳{\mathcal{X}} is merely dlt, each stratum YY is normal, and comes with a canonically defined effective ℚ{\mathbb{Q}}-divisor BYB_{Y} such that (Y,BY)(Y,B_{Y}) is dlt and still satisfies (5.6) (cf. [Kol13, 4.19]). We have

BY=∑i∉JYEi∩Y+BY′B_{Y}=\sum_{i\notin J_{Y}}E_{i}\cap Y+B^{\prime}_{Y}

where BY′B^{\prime}_{Y} is an effective ℚ{\mathbb{Q}}-divisor supported in the complement of 𝒳snc{\mathcal{X}}_{\mathrm{snc}}.

Example 5.11.

For each ii, Ei∩(𝒳∖𝒳snc)E_{i}\cap({\mathcal{X}}\setminus{\mathcal{X}}_{\mathrm{snc}}) contains finitely many prime divisors Fi​kF_{ik} of EiE_{i}. At the generic point of Fi​kF_{ik}, 𝒳{\mathcal{X}} has cyclic quotient singularities, and

BEi=∑j≠iEj∩Ei+∑k(1−1mi​k)​Fi​kB_{E_{i}}=\sum_{j\neq i}E_{j}\cap E_{i}+\sum_{k}\left(1-\frac{1}{m_{ik}}\right)F_{ik}

with mi​km_{ik} the order of the corresponding cyclic groups, cf. [Kol13, 3.36.3].

Now let ψ\psi be a model metric on KXanK_{X}^{\mathrm{an}}, determined by a model ℒ{\mathcal{L}} of KXK_{X} on a proper dlt model 𝒳{\mathcal{X}} of XX. Introduce as before the function κ:=AX−ψ\kappa:=A_{X}-\psi on XanX^{\mathrm{an}}, and note that the ℚ{\mathbb{Q}}-Cartier divisor

D:=K𝒳/Slog−ℒ−κmin​𝒳0=∑i(κ⁡(vEi)−κmin)​bi​EiD:=K^{\mathrm{log}}_{{\mathcal{X}}/S}-{\mathcal{L}}-\kappa_{\min}{\mathcal{X}}_{0}=\sum_{i}(\kappa(v_{E_{i}})-\kappa_{\min})b_{i}E_{i}

is effective.

Lemma 5.12.

If YY is a stratum of 𝒳0{\mathcal{X}}_{0} corresponding to a face σ\sigma of Δ⁡(ℒ)\Delta({\mathcal{L}}), then Y⊄supp⁡DY\not\subset\operatorname{supp}D. It follows that the ℚ{\mathbb{Q}}-Cartier divisor

BYℒ:=BY−D|YB^{\mathcal{L}}_{Y}:=B_{Y}-D|_{Y}

is well-defined, and we have a canonical identification ℒ|Y=K(Y,BYℒ){\mathcal{L}}|_{Y}=K_{(Y,B^{\mathcal{L}}_{Y})} as ℚ{\mathbb{Q}}-line bundles. Further, if σ\sigma is a maximal face of Δ⁡(ℒ)\Delta({\mathcal{L}}), then the pair (Y,BYℒ)(Y,B^{\mathcal{L}}_{Y}) is subklt.

We emphasize that BYℒB^{\mathcal{L}}_{Y} is not effective in general.

Proof.

The first two points are clear. When σ\sigma is a maximal face, each EiE_{i} meeting YY satisfies κ⁡(vEi)>κmin\kappa(v_{E_{i}})>\kappa_{\min}. As a result, D|YD|_{Y} contains each lc center Ei∩YE_{i}\cap Y of (Y,BY)(Y,B_{Y}), which yields the last assertion. ∎

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