ScalingStacks

Example 3.12 . [033V]

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Example 3.12.

Assume again X=ℂ​ℙnX=\mathbb{C}\mathbb{P}^{n} and ω\omega is the Fubini-Study Kähler form. Consider the totally real subspace ℝ​ℙn\mathbb{R}\mathbb{P}^{n} of points with real coordinates (the closure of ℝn⊂ℂn\mathbb{R}^{n}\subset\mathbb{C}^{n} in ℂ​ℙn\mathbb{C}\mathbb{P}^{n}). Then

12​(1+2)≤Tω​(ℝ​ℙn)≤1.\frac{1}{2(1+\sqrt{2})}\leq T_{\omega}(\mathbb{R}\mathbb{P}^{n})\leq 1.

Indeed set Bℝn:=ℝn∩𝔹nB_{\mathbb{R}^{n}}:=\mathbb{R}^{n}\cap\mathbb{B}^{n}. It follows from the discussion above that

Tω​(ℝ​ℙn)≥12​T𝔹n​(𝔹ℝn).T_{\omega}(\mathbb{R}\mathbb{P}^{n})\geq\frac{1}{2}T_{\mathbb{B}^{n}}(\mathbb{B}_{\mathbb{R}^{n}}).

Now there is an explicit formula for LBℝn∗L_{B_{\mathbb{R}^{n}}}^{*} (Lundin’s formula, see [27]),

LBℝn∗(z)=sup{log+|h(<z,ξ>)|/||ξ||=1},z∈ℂn,L_{B_{\mathbb{R}^{n}}}^{*}(z)=\sup\{\log^{+}|h(<z,\xi>)|\,/||\xi||=1\},\;z\in\mathbb{C}^{n},

where h⁡(ζ)=ζ+ζ2−1h(\zeta)=\zeta+\sqrt{\zeta^{2}-1}. A simple computation yields |h⁡(ζ)|≤log⁡[|z|+|z|2+1]|h(\zeta)|\leq\log[|z|+\sqrt{|z|^{2}+1}] for ζ=<z,ξ>\zeta=<z,\xi> with ‖ξ‖=1||\xi||=1. We infer

L𝔹ℝn​(z)≤log⁡[|z|+|z|2+1]≤log⁡[1+2]​ in ​𝔹n,L_{\mathbb{B}_{\mathbb{R}^{n}}}(z)\leq\log\left[|z|+\sqrt{|z|^{2}+1}\right]\leq\log[1+\sqrt{2}]\;\text{ in }\mathbb{B}^{n},

which yields the desired inequality.

Observe that the minorant is independent of the dimension nn. This has been used recently in complex dynamics by Dinh and Sibony [18].

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