ScalingStacks

Proof. [03EC]

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Proof.

According to Proposition 3.2 we can choose ss big enough so that the Logs\mathrm{Log}_{s}-image of the affine part of every hypersurface Hsw​(a)H_{s}^{w}(a) lies in the ϵ\epsilon-neighborhood of the Minkowski sum Vwδ+cone⁡(w)V^{\delta}_{w}+\operatorname{cone}(w). Also, recall from Lemma 3.3 that Vwδ+cone⁡(w)V^{\delta}_{w}+\operatorname{cone}(w) lies in the domain Q({0}∣w⟂)λ​(ϵ)Q^{\lambda}_{(\{0\}\mid w^{\perp})}(\epsilon). Thus, we can assume that the affine parts of all hypersurfaces Hsw​(a)H_{s}^{w}(a) lie in Logs−1​(Q({0}∣w⟂)λ​(ϵ))\mathrm{Log}_{s}^{-1}(Q^{\lambda}_{(\{0\}\mid w^{\perp})}(\epsilon)).

We choose a basis {ei}\{e_{i}\} of (ℤd)∗(\mathbb{Z}^{d})^{*} such that

⟨e1,w⟩=−1 and ⟨ei,w⟩=0,i=2,…,d.\langle e_{1},w\rangle=-1\text{ and }\langle e_{i},w\rangle=0,i=2,\dots,d.

Then the affine coordinate functions yi=xeiy_{i}=x^{e_{i}} can be extended (by allowing zero values for y1y_{1}) to the open part of the toric divisor ZwZ_{w} corresponding to the facet Fw⊂∂ΔνF_{w}\subset\partial{\Delta_{\nu}}. Moreover, in these coordinates the preimage of each flow line ℱq\mathcal{F}_{q} in (ℂ\{0})d(\mathbb{C}\backslash\{0\})^{d} is defined by fixing the values of |y2|,…,|yd||y_{2}|,\dots,|y_{d}|, so that its closure Xs​(q)X_{s}(q) is defined by the same equations, but allowing the zero value for y1y_{1}. Hence, we can use {yi}\{y_{i}\} as global coordinates on Xs​(Vwδ)X_{s}(V^{\delta}_{w}).

Multiplying the affine equation of Hsw​(a)H_{s}^{w}(a) by y1y_{1} we note that the Laurent polynomial

y1​∑m∈Gwsλ⁡(m)​xm+y1​∑m∉Gw∪{0}am​sλ⁡(m)​xmy_{1}\sum\limits_{m\in G_{w}}s^{\lambda(m)}x^{m}+y_{1}\sum\limits_{m\notin G_{w}\cup\{0\}}a_{m}s^{\lambda(m)}x^{m}

has only positive powers of y1y_{1}, where as its first part P1​(y)=y1​∑m∈Gwsλ⁡(m)​xmP_{1}(y)=y_{1}\sum\limits_{m\in G_{w}}s^{\lambda(m)}x^{m} is independent of y1y_{1} at all. Thus, we get the global equation for the family in Xs​(Vwδ)X_{s}(V^{\delta}_{w}).

Now we can repeat the estimates for the monomials and their log-derivatives. Whenever Logs​(x)∈Q({0}∣w⟂)λ​(ϵ)\mathrm{Log}_{s}(x)\in Q^{\lambda}_{(\{0\}\mid w^{\perp})}(\epsilon), we have

⟨m,log⁡|x|log⁡|s|⟩+λ⁡(m)≤λ⁡(0)−ϵ, for all ​m∉Gw∪{0},\langle m,\frac{\log|x|}{\log|s|}\rangle+\lambda(m)\leq\lambda(0)-\epsilon,\text{ for all }m\notin G_{w}\cup\{0\},

or, equivalently,

|xm​sλ​(m)|≤|s|−ϵ​|s|λ⁡(0).|x^{m}s^{\lambda}(m)|\leq|s|^{-\epsilon}|s|^{\lambda(0)}.

When written in the yy-coordinates these estimates extends by continuity from the affine part to the entire Xs​(Vwδ)X_{s}(V^{\delta}_{w}).

To see that Hsw​(a)H_{s}^{w}(a) has no critical points we differentiate its defining equation with respect to y1y_{1}:

∂∂y1​(y1​sλ⁡(0)+y1​∑m∈Gwsλ⁡(m)​xm+y1​∑m∉Gw∪{0}am​sλ⁡(m)​xm)=sλ⁡(0)+(1+y1​∂∂y1)​∑m∉Gw∪{0}am​sλ⁡(m)​xm=sλ⁡(0)​(1+O⁡(|s|−ϵ))≠0,\frac{\partial}{\partial y_{1}}\bigl(y_{1}s^{\lambda(0)}+y_{1}\sum\limits_{m\in G_{w}}s^{\lambda(m)}x^{m}+y_{1}\sum\limits_{m\notin G_{w}\cup\{0\}}a_{m}s^{\lambda(m)}x^{m}\bigr)\\ =s^{\lambda(0)}+\bigl(1+y_{1}\frac{\partial}{\partial y_{1}}\bigr)\sum\limits_{m\notin G_{w}\cup\{0\}}a_{m}s^{\lambda(m)}x^{m}=s^{\lambda(0)}(1+O(|s|^{-\epsilon}))\neq 0,

for large enough ss.

Finally, Lemma 3.6 asserts that the vector field 𝔛\mathfrak{X} in ⋃q∈Vwδℱq\bigcup_{q\in V^{\delta}_{w}}\mathcal{F}_{q} is constant and equal to ww. It means that ∂∂y1\frac{\partial}{\partial y_{1}} is a tangent vector to Xs​(q)X_{s}(q), q∈Vwδq\in V^{\delta}_{w}, and it is transversal to Hsw​(a)H_{s}^{w}(a) by the above calculation. ∎

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