ScalingStacks

Proof. [05B9]

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Proof.

We already know by Theorem 5.2 that the equation holds on the set of vertices. Furthermore it is clear from the definition, that c1​(𝒪¯h∘p𝔛′)nc_{1}\left(\overline{\mathcal{O}}^{h\circ p_{\mathfrak{X}^{\prime}}}\right)^{n} is supported on the vertices of 𝔇\mathfrak{D}. What remains to show is that this also holds for MA⁡(h)\MA(h).

Let U:=τ∖{u∈τ|u​ is a vertex of ​𝔇}U:=\tau\setminus\left\{u\in\tau\;\Big|\;u\text{ is a vertex of }\mathfrak{D}\right\}. We want to show MA⁡(h)​(U)=0\MA(h)(U)=0. Let Δ1,…,Δr\Delta_{1},...,\Delta_{r} be the open faces of 𝔇\mathfrak{D} of dimension at least one. For every j∈{1,…,r}j\in\{1,...,r\} there is a vj∈ℝn∖{0}v_{j}\in\mathbb{R}^{n}\setminus\{0\} such that for all y∈Δjy\in\Delta_{j} there exists ϵ∈ℝ+\epsilon\in\mathbb{R}_{+} such that y±ϵ​vj∈Δjy\pm\epsilon v_{j}\in\Delta_{j}. Furthermore hj:=h|Δj=𝒎j​𝒙+v⁡(αj)h_{j}:=h\Big|_{\Delta_{j}}=\boldsymbol{m}_{j}\boldsymbol{x}+v(\alpha_{j}) for some 𝒎j∈ℤn\boldsymbol{m}_{j}\in\mathbb{Z}^{n} and αj∈K×\alpha_{j}\in K^{\times} and we define hjl​i​n:=𝒎j​𝒙h_{j}^{lin}:=\boldsymbol{m}_{j}\boldsymbol{x}. Now let y∈Uy\in U. Then there is an ii such that y∈Δiy\in\Delta_{i}. For p∈∇h​(y)p\in\nabla h(y) and ϵ\epsilon as above it follows

ϵ​⟨vi,p⟩\displaystyle\epsilon\langle v_{i},p\rangle =hi​(y)+⟨y+ϵ​vi−y,p⟩−hi​(y)\displaystyle=h_{i}(y)+\langle y+\epsilon v_{i}-y,p\rangle-h_{i}(y)
≤hi​(y+ϵ​vi)−hi​(y)\displaystyle\leq h_{i}(y+\epsilon v_{i})-h_{i}(y)
=hil​i​n​(ϵ​vi)\displaystyle=h_{i}^{lin}(\epsilon v_{i})
=ϵ​hil​i​n​(vi),\displaystyle=\epsilon h_{i}^{lin}(v_{i}),

hence

⟨vi,p⟩≤hil​i​n​(vi).\langle v_{i},p\rangle\leq h_{i}^{lin}(v_{i}).

A similar argument shows

−ϵ⁡⟨vi,p⟩≤−ϵ​hil​i​n​(vi)-\epsilon\langle v_{i},p\rangle\leq-\epsilon h_{i}^{lin}(v_{i})

and hence

⟨vi,p⟩≥hil​i​n​(vi).\langle v_{i},p\rangle\geq h_{i}^{lin}(v_{i}).

We conclude ⟨vi,p⟩=hil​i​n​(vi)\langle v_{i},p\rangle=h_{i}^{lin}(v_{i}) and pp lies in a hypersurface which depends on ii but not on yy. Hence ⋃y∈U∇h​(y)\bigcup_{y\in U}\nabla h(y) is contained in the union of rr hypersurfaces. Therefore

MA⁡(h)​(U)=λ⁡(⋃y∈U∇h​(y))=0,\MA(h)(U)=\lambda\left(\bigcup_{y\in U}\nabla h(y)\right)=0,

∎

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