ScalingStacks

Example 3.3 . [028N]

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Example 3.3.

Let X⊂ℂ2X\subset{\mathbb{C}}^{2} be the irreducible cubic with equation x​y=x3+1xy=x^{3}+1. Then

X¯={[t:x:y]∈ℙ2:xyt=x3+t3},X¯=X∪{a},a=[0:0:1].\overline{X}=\{[t:x:y]\in{\mathbb{P}}^{2}:\,xyt=x^{3}+t^{3}\},\;\overline{X}=X\cup\{a\},\;a=[0:0:1].

The germ (X¯,a)(\overline{X},a) has two irreducible components X1,X2X_{1},\,X_{2}, both are smooth at aa, X1X_{1} being tangent to the line {x=0}\{x=0\}, and X2X_{2} to the line {t=0}\{t=0\}.

Note that in fact X⊂ℂ⋆×ℂX\subset{\mathbb{C}}^{\star}\times{\mathbb{C}} is the graph of the rational function y=x2+x−1y=x^{2}+x^{-1}, x∈ℂ⋆x\in{\mathbb{C}}^{\star}. If (x,y)∈X(x,y)\in X and x→0x\to 0 then (x,y)→a(x,y)\to a along X1X_{1}, while as x→∞x\to\infty then (x,y)→a(x,y)\to a along X2X_{2}. The function

u⁡(x,y)=max⁡{−log⁡|x|,2​log⁡|x|+1}u(x,y)=\max\{-\log|x|,2\log|x|+1\}

is psh in ℂ⋆×ℂ{\mathbb{C}}^{\star}\times{\mathbb{C}}. It is easy to check that η:=u|X∈ℒ(X)\eta:=u\,|_{{}_{X}}\in{\mathcal{L}}(X) and

lim supX1∋[1:ζ]→a(η(ζ)−ρ(1,ζ))=0,lim supX2∋[1:ζ]→a(η(ζ)−ρ(1,ζ))=1.\limsup_{X_{1}\ni[1:\zeta]\to a}(\eta(\zeta)-\rho(1,\zeta))=0\;,\;\;\limsup_{X_{2}\ni[1:\zeta]\to a}(\eta(\zeta)-\rho(1,\zeta))=1.

Hence η\eta does not admit an extension in ℒ⁡(ℂ2){\mathcal{L}}({\mathbb{C}}^{2}).

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