ScalingStacks

4.7 [035Q]

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4.7

The tropical weight mm on an nn-dimensional polyhedron σ\sigma of Trop⁡(Y){\rm Trop}(Y) is defined in the following way. By density of the value group Γ\Gamma in ℝ{\mathbb{R}}, there is ω∈Γr∩relint⁡(σ)\omega\in\Gamma^{r}\cap{\rm relint}(\sigma). We choose t∈𝔾mr​(K)t\in{\mathbb{G}}_{m}^{r}(K) with trop⁡(t)=ω{\rm trop}(t)=\omega. Then the closure of t−1​Yt^{-1}Y in (𝔾mr)K∘({{\mathbb{G}}_{m}^{r}})_{{K^{\circ}}} is a flat variety over K∘{K^{\circ}} whose special fibre is called the initial degeneration inω​(Y){\rm in}_{\omega}(Y) of YY at ω\omega. Note that inω​(Y){\rm in}_{\omega}(Y) is a closed subscheme of (𝔾mr)K~({\mathbb{G}}_{m}^{r})_{{\tilde{K}}}. Let mWm_{W} be the multiplicity of the irreducible component WW of inω​(Y){\rm in}_{\omega}(Y). Then the tropical weight mσm_{\sigma} is defined by mσ:=∑WmWm_{\sigma}:=\sum_{W}m_{W}, where WW ranges over all irreducible components of inω​(Y){\rm in}_{\omega}(Y). One can show that the definition is independent of the choices of ω\omega and tt. It is a non-trivial fact from tropical geometry that (Trop⁡(Y),m)({\rm Trop}(Y),m) is a tropical cycle (see [Gu12], §13, for details).

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