ScalingStacks

4.10 [035U]

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4.10

If f:X1→X2f:X_{1}\rightarrow X_{2} is a morphism of varieties over KK, then we define the push-forward of X1X_{1} with respect to ff as the cycle f∗​(X1):=deg⁡(f)​f⁡(X1)¯f_{*}(X_{1}):=\deg(f)\overline{f(X_{1})}, where the degree of ff is defined as deg(f):=[K(X1):K(f(X1))]\deg(f):=[K(X_{1}):K(f(X_{1}))] if ff is generically finite and we set deg⁡(f):=0\deg(f):=0 if [K(X1):K(f(X1))]=∞[K(X_{1}):K(f(X_{1}))]=\infty. By restriction, the push-forward can be defined in the same way on prime cycles of X1X_{1} and extends by linearity to all cycles of X1X_{1}.

Now let φ:U→T=𝔾mr\varphi:U\rightarrow T={\mathbb{G}}_{m}^{r} be a moment map of the open subset UU of XX. By 4.6,

Trop⁡(φ∗​(U)):=deg⁡(φ)​Trop​(φ⁡(U)¯){\rm Trop}(\varphi_{*}(U)):=\deg(\varphi){\rm Trop}(\overline{\varphi(U)})

is a tropical cycle on ℝr{\mathbb{R}}^{r}. If φ\varphi is generically finite, then this tropical cycle is of pure dimension dim(X)\dim(X) and the support is equal to φtrop​(Uan){\varphi_{\rm trop}}({U^{\rm an}}) (see Lemma 4.9).

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