ScalingStacks

7.4 [037G]

Original official author HTML, exact retained edition. Historical TeX conversion verdicts remain unchanged. Cited-edition alignment and mathematical self-containment are not assessed.

Complete original source context · Original author HTML

7.4

We consider now a compact analytic space ZZ over KK of pure dimension nn. Theorem 7.3 shows that the tropical variety φtrop​(Z){\varphi_{\rm trop}}(Z) is the support of an integral Γ\Gamma-affine polytopal complex in NℝN_{\mathbb{R}}. Our next goal is to endow this complex with canonical tropical multiplicities. This will lead to the definition of a weighted polytopal complex (φtrop)∗​(cyc⁡(Z))({\varphi_{\rm trop}})_{*}({\rm cyc}(Z)) which is canonical up to subdivision.

If dim(φtrop​(Z))<n\dim({\varphi_{\rm trop}}(Z))<n, then we set (φtrop)∗​(cyc⁡(Z))=0({\varphi_{\rm trop}})_{*}({\rm cyc}(Z))=0 meaning that we choose all tropical weights equal to zero. It remains to consider the case dim(φtrop​(Z))=n\dim({\varphi_{\rm trop}}(Z))=n. As seen in Example 4.5, we may identify ℝr{\mathbb{R}}^{r} with the skeleton S⁡(Tan)S({T^{\rm an}}) of Tan{T^{\rm an}}. We choose a generic surjective homomorphism q:T→T′q:T\rightarrow T^{\prime} onto a split multiplicative torus T′=Spec⁡(K⁡[M′])T^{\prime}={\rm Spec}(K[M^{\prime}]) of rank n=dim(Z)n=\dim(Z). Generic means that the corresponding linear map Trop⁡(q){\rm Trop}(q) is injective on every polytope contained in φtrop​(Z){\varphi_{\rm trop}}(Z). By Theorem 7.3, there is an integral Γ\Gamma-affine polytopal complex 𝒞{\mathscr{C}} in NℝN_{\mathbb{R}} with |𝒞|=φtrop​(Z)|{\mathscr{C}}|={\varphi_{\rm trop}}(Z) such that Trop​(q)​(τ){\rm Trop}(q)(\tau) is disjoint from (q∘φ)trop​(∂Z)(q\circ\varphi)_{\rm trop}(\partial Z) for every nn-dimensional face σ\sigma of 𝒞{\mathscr{C}} and τ:=relint⁡(σ)\tau:={\rm relint}(\sigma). By passing to a subdivision, we may assume that Trop​(q)∗​(𝒞){\rm Trop}(q)_{*}({\mathscr{C}}) is a polyhedral complex in Nℝ′N_{\mathbb{R}}^{\prime} as in 3.9.

We identify Trop​(q)​(τ)⊂ℝn{\rm Trop}(q)(\tau)\subset{\mathbb{R}}^{n} with a subset of the skeleton S⁡((T′)an)S((T^{\prime})^{\rm an}) as in Remark 4.5. Then it is clear that q∘φq\circ\varphi restricts to a map φtrop−1​(τ)→Trop⁡(q)​(τ)\varphi_{\rm trop}^{-1}(\tau)\rightarrow{\rm Trop}(q)(\tau) which agrees with Trop⁡(q)∘φtrop{\rm Trop}(q)\circ{\varphi_{\rm trop}} on φtrop−1​(τ)\varphi_{\rm trop}^{-1}(\tau) using the identification S⁡((T′)an)=ℝnS((T^{\prime})^{\rm an})={\mathbb{R}}^{n}. It is shown in [CD12], §2.4, that φtrop−1​(τ)→Trop⁡(q)​(τ)\varphi_{\rm trop}^{-1}(\tau)\rightarrow{\rm Trop}(q)(\tau) is a finite flat and surjective morphism which means that every point pp of Trop​(q)​(τ){\rm Trop}(q)(\tau) has a neighbourhood W′W^{\prime} in (T′)an(T^{\prime})^{\rm an} such that (q∘φ)−1​(W′)→W′(q\circ\varphi)^{-1}(W^{\prime})\rightarrow W^{\prime} has these properties. Since τ\tau is connected, the corresponding degree depends only on τ\tau and not on the choice of pp. We denote this degree by [φtrop−1(τ):Trop(q)(τ)][\varphi_{\rm trop}^{-1}(\tau):{\rm Trop}(q)(\tau)].

Recall that NσN_{\sigma} is the canonical lattice in the affine space generated by σ\sigma. Then the character lattice M′M^{\prime} of T′T^{\prime} is of finite index in Mσ=Hom⁡(Nσ,ℤ)M_{\sigma}={\rm Hom}(N_{\sigma},{\mathbb{Z}}).

Original mathematics by the credited authors. Source-backed reader collection; mathematical self-containment is not assessed.