ScalingStacks

5.6. Proof of Theorems 2 and 4 [04TF]

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5.6. Proof of Theorems 2 and 4

Here we prove that VtV_{t} is non-singular and that λt\lambda_{t} satisfies to all hypotheses of Theorem 3 for a large t>0t>0.

Note that ‖t−v⁡(j)​zj‖=tj​x−v⁡(j)||t^{-v(j)}z^{j}||=t^{jx-v(j)}, where j​x∈ℝjx\in\mathbb{R} stands for the scalar product, at any zz such that Logt=x\operatorname{Log}_{t}=x. Let F⊂ΠF\subset\Pi be an open (n+2−k)(n+2-k)-cell.

Lemma 5.5.

There exists kk monomials t−v⁡(j1)​zj1,…,t−v⁡(jk)​zjkt^{-v(j_{1})}z^{j_{1}},\dots,t^{-v(j_{k})}z^{j_{k}} that dominate ftf_{t} in a neighborhood of FF. Namely, any other monomial evaluated at a point near FF has a smaller order by tt. Furthermore, the hypersurface

∑m=1kt−v⁡(jm)​zjm=0\sum\limits_{m=1}^{k}t^{-v(j_{m})}z^{j_{m}}=0

is isomorphic to the hyperplane z1+⋯+zk−1+1=0z_{1}+\dots+z_{k-1}+1=0 under the multiplicative change of coordinates by an element of S​Ln+1​(ℤ)SL_{n+1}(\mathbb{Z}).

Proof.

This follows from the maximality of Π\Pi. By Proposition 1.1 FF is dual to a kk-dimensional polyhedron from a subdivision of Δ\Delta. Since Π\Pi is maximal, this polyhedron is the standard (k−1)(k-1)-simplex up to action of S​Ln+1​(ℤ)SL_{n+1}(\mathbb{Z}). ∎

This lemma implies that Vt∘{V}^{\circ}_{t} is non-singular for large t>0t>0. Indeed, it is covered by a finite number of open sets and in each set it is a small perturbation of a hyperplane. Furthermore, its compactification Vt⊂ℂ​TΔV_{t}\subset\mathbb{C}T_{\Delta} is smooth and transverse to the coordinate hyperplanes as the same reasoning with the terms of smaller order applies to the affine charts of ℂ​TΔ\mathbb{C}T_{\Delta}.

Our next step is to isotop VtV_{t} over NjN_{j} as in 3.4. Recall that NjN_{j} was defined in 5.5 as a small neighborhood of Uj¯⊂Π\bar{U_{j}}\subset\Pi in ℝn+1\mathbb{R}^{n+1}. Denote

Qjn=Mj−1​(Ht​(Qn))∩Logt−1⁡(Nj).Q^{n}_{j}=M_{j}^{-1}(H_{t}(Q^{n}))\cap\operatorname{Log}_{t}^{-1}(N_{j}).

By the last conclusion of Proposition 3.6 these manifolds coincide over Nj∩NkN_{j}\cap N_{k} for t>>0t>>0. We set

QΠ=⋃jQjn.Q_{\Pi}=\bigcup\limits_{j}Q^{n}_{j}.

Note that for t>>0t>>0 Vt∘{V}^{\circ}_{t} is isotopic to QΠQ_{\Pi} by the same isotopy as in the proof of Proposition 3.6 since all other monomials of ftf_{t} have smaller order in tt. This proves Theorem 4. As in Proposition 3.6 the closure QΠ¯⊂ℂ​TΔ\bar{Q_{\Pi}}\subset\mathbb{C}T_{\Delta} is a smooth manifold. Similarly, VtV_{t} is isotopic to QΠ¯\bar{Q_{\Pi}} in ℂ​TΔ\mathbb{C}T_{\Delta}.

In the proof of Theorem 3 we may assume that V∘=Vt∘{V}^{\circ}={V}^{\circ}_{t} since its closure Vt⊂ℂ​TΔV_{t}\subset\mathbb{C}T_{\Delta} is smooth and transverse to the coordinate hyperplanes. Similarly, in the proof of Theorems 1, 1’ and 2 we may assume that V=VtV=V_{t}. We define λ∘:V∘→Π\lambda^{\circ}:{V}^{\circ}\to\Pi as a composition of the isotopy V≈QΠV\approx Q_{\Pi} (note that since this map is realized by an ambient isotopy it is a symplectomorphism by Moser’s trick), the map Logt:(ℂ∗)n+1→ℝn+1\operatorname{Log}_{t}:(\mathbb{C}^{*})^{n+1}\to\mathbb{R}^{n+1} and the projection πℱΠ:𝒩→Π\pi_{\mathcal{F}_{\Pi}}:\mathcal{N}\to\Pi. To define λ:V→Π¯\lambda:V\to\bar{\Pi} we compactify the previous construction by using QΠ¯\bar{Q_{\Pi}} and the reparametrized moment map to Δ\Delta as in 1.3.

Refer to caption

Figure 8. The amoeba of the localization QΠQ_{\Pi} of a hypersurface.

This proves Theorem 2, since everything in our construction is equivariant with respect to complex conjugation as long as aja_{j} in the patchworking polynomial are real. The fibration λ\lambda is totally real since it is totally real for a hyperplane.

Also, by Proposition 3.6 this proves the second and the third conclusions in Theorems 1, 1’ and 3. The homotopy type of Π¯\bar{\Pi} and Π\Pi is the wedge of pgp_{g} copies of SnS^{n}, where pg=hn,0p_{g}=h^{n,0} by Proposition 1.10.

To finish the proof of Theorems 1. 1’ and 3 we need to prove injectivity of the induced homomorphism in cohomology and to exhibit the Lagrangian spheres lifting the cycles from Π\Pi.

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