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5.1. Josefson’s theorem [0346]

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5.1. Josefson’s theorem

In this section we assume that ω\omega is Kähler and normalized by V​o​lω​(X)=1Vol_{\omega}(X)=1. We first prove an inequality relating TωT_{\omega} and C​a​pωCap_{\omega}. We do not know if a reverse inequality holds as it is the case in the local theory [2]. Then we prove (theorem 5.2) a quantitative version of Josefson’s theorem that every locally pluripolar set is actually P​S​H​(X,ω)PSH(X,\omega)-polar. In the local theory this result is due to El Mir [19]. We follow the approach of Alexander-Taylor [2].

Proposition 5.1.

Let KK be a compact subset of XX. If supXVK,ω≤1\sup_{X}V_{K,\omega}\leq 1 then C​a​pω​(K)=C​a​pω​(X)=1Cap_{\omega}(K)=Cap_{\omega}(X)=1. If supXVK,ω≥1\sup_{X}V_{K,\omega}\geq 1 then

0≤Tω(K)≤exp(−Capω(K)−1/n).0\leq T_{\omega}(K)\leq\exp(-Cap_{\omega}(K)^{-1/n}).
Proof.

Set MK=supXVK,ωM_{K}=\sup_{X}V_{K,\omega}. If MK=+∞M_{K}=+\infty then KK is P​S​H​(X,ω)PSH(X,\omega)-polar (theorem 3.2) and there is nothing to prove: Tω​(K)=C​a​pω​(K)=0T_{\omega}(K)=Cap_{\omega}(K)=0. So we assume in the sequel MK<+∞M_{K}<+\infty hence VK,ω∗∈P​S​H​(X,ω)V_{K,\omega}^{*}\in PSH(X,\omega). If MK≥1M_{K}\geq 1 then uK:=MK−1​VK,ω∗∈P​S​H​(X,ω)u_{K}:=M_{K}^{-1}V_{K,\omega}^{*}\in PSH(X,\omega) with 0≤uK≤10\leq u_{K}\leq 1 on XX. Since ωVK,ω∗≤MK​ωuK\omega_{V_{K,\omega}^{*}}\leq M_{K}\omega_{u_{K}}, we get

1MKn=1MKn​∫K(ωVK,ω∗)n≤∫K(ωuK)n≤C​a​pω​(K)\frac{1}{M_{K}^{n}}=\frac{1}{M_{K}^{n}}\int_{K}(\omega_{V_{K,\omega}^{*}})^{n}\leq\int_{K}(\omega_{u_{K}})^{n}\leq Cap_{\omega}(K)

whence Tω(K)≤exp(−Capω(K)−1/n)T_{\omega}(K)\leq\exp(-Cap_{\omega}(K)^{-1/n}).

If 0≤MK≤10\leq M_{K}\leq 1 then 0≤VK,ω∗≤10\leq V_{K,\omega}^{*}\leq 1 hence

1=∫K(ωVK,ω∗)n≤C​a​pω​(K)≤C​a​pω​(X)=1.1=\int_{K}(\omega_{V_{K,\omega}^{*}})^{n}\leq Cap_{\omega}(K)\leq Cap_{\omega}(X)=1.

∎

It follows from the previous proposition and corollary 2.8 that ω\omega-psh functions are quasicontinuous with respect to the capacity TωT_{\omega}.

Question. Is there -as in the local context [2]- a constant A>0A>0 s.t.

Tω(⋅)≥exp(−A/Capω(⋅))?T_{\omega}(\cdot)\geq\exp(-A/Cap_{\omega}(\cdot))\;?
Theorem 5.2.

Locally pluripolar sets are P​S​H​(X,ω)PSH(X,\omega)-polar.

Proof.

More precisely we are going to show the following: consider Ω\Omega an open subset of XX, v∈P​S​H−​(Ω)v\in PSH^{-}(\Omega) and P⊂{v=−∞}P\subset\{v=-\infty\}. Fix 0<ε<1/n0<\varepsilon<1/n and Vt:=VGt,ωV_{t}:=V_{G_{t},\omega} where Gt={x∈Ω/v(x)<−t}G_{t}=\{x\in\Omega\,/\,v(x)<-t\}. Then

φε​(x):=1ε​∫1+∞1t1+ε​[Vt​(x)−supXVt]​𝑑t\varphi_{\varepsilon}(x):=\frac{1}{\varepsilon}\int_{1}^{+\infty}\frac{1}{t^{1+\varepsilon}}[V_{t}(x)-\sup_{X}V_{t}]dt

is a ω\omega-psh function such that P⊂{φε=−∞}P\subset\{\varphi_{\varepsilon}=-\infty\}.

Indeed since GtG_{t} is open, we have Vt∈P​S​H​(X,ω)V_{t}\in PSH(X,\omega) and Vt=0V_{t}=0 on GtG_{t} (see proposition 3.6). Observe that φε\varphi_{\varepsilon} is a sum of negative ω\omega-psh functions hence it is either identically −∞-\infty or a well defined A​ωA\omega-psh function with A=ε−1​∫1+∞t−(1+ε)​𝑑t=1A=\varepsilon^{-1}\int_{1}^{+\infty}t^{-(1+\varepsilon)}dt=1. Recall that −C+supXVt≤∫XVt​ωn≤supXVt-C+\sup_{X}V_{t}\leq\int_{X}V_{t}\omega^{n}\leq\sup_{X}V_{t} (proposition 1.7). Therefore ∫Xφε​ωn≥−C\int_{X}\varphi_{\varepsilon}\omega^{n}\geq-C hence φε∈P​S​H​(X,ω)\varphi_{\varepsilon}\in PSH(X,\omega).

Fix x∈Ωx\in\Omega such that v⁡(x)<−1v(x)<-1. Observe that Vt−supXVt≤0V_{t}-\sup_{X}V_{t}\leq 0 with Vt​(x)=0V_{t}(x)=0 if x∈Gtx\in G_{t}, i.e. when |v⁡(x)|>t|v(x)|>t. Therefore

φε(x)≤−1ε∫1|v⁡(x)|supXVtt1+εdt.\varphi_{\varepsilon}(x)\leq-\frac{1}{\varepsilon}\int_{1}^{|v(x)|}\frac{\sup_{X}V_{t}}{t^{1+\varepsilon}}dt.

Recall now that C​a​pωCap_{\omega} is always dominated by C​a​pB​TCap_{BT} hence C​a​pω​(Gt)≤C1/t<1Cap_{\omega}(G_{t})\leq C_{1}/t<1 if tt is large enough. We infer from the previous proposition that

−supXVt≤−[Capω(Gt)]−1/n≤−C2t1/n,-\sup_{X}V_{t}\leq-[Cap_{\omega}(G_{t})]^{-1/n}\leq-C_{2}t^{1/n},

which yields

φε​(x)≤−C2ε​∫1|v⁡(x)|d​tt1+ε−1/n≤−C3​|v⁡(x)|1/n−ε+C4.\varphi_{\varepsilon}(x)\leq\frac{-C_{2}}{\varepsilon}\int_{1}^{|v(x)|}\frac{dt}{t^{1+\varepsilon-1/n}}\leq-C_{3}|v(x)|^{1/n-\varepsilon}+C_{4}.

Note that φε​(x)=−∞\varphi_{\varepsilon}(x)=-\infty whenever v⁡(x)=−∞v(x)=-\infty hence P⊂{φε=−∞}P\subset\{\varphi_{\varepsilon}=-\infty\}. ∎

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