Math 2.0

Several complex variables and analytic spaces Definition

Pluripolar set

Builds on Plurisubharmonic function

Definition

Let E⊆ℂnE \subseteq \C^n. The set EE is pluripolar if each point a∈Ea \in E has an open neighbourhood UU with the following property. There is a plurisubharmonic function uu on UU, not identically −∞-\infty on any component of UU, such that E∩U⊆{z∈U:u(z)=−∞}.E \cap U \subseteq \set{z \in U : u(z) = -\infty}. The set EE is complete pluripolar in an open set Ω\Omega if E={z∈Ω:u(z)=−∞}E = \set{z \in \Omega : u(z) = -\infty} for some function uu of the following kind. The function uu is plurisubharmonic on Ω\Omega and not identically −∞-\infty on any component of Ω\Omega.

Remarks

Some authors call the sets of the first definition locally pluripolar, and call EE globally pluripolar if one function uu plurisubharmonic on all of ℂn\C^n, not identically −∞-\infty, has E⊆{u=−∞}E \subseteq \set{u = -\infty}. By Josefson's theorem the two notions agree. Example: E={z∈ℂ2:z1=0}E = \set{z \in \C^2 : z_1 = 0} is pluripolar; take U=ℂ2U = \C^2 and u(z)=log⁡∣z1∣u(z) = \log\abs{z_1}, which is plurisubharmonic and equals −∞-\infty exactly on EE. Non-example: ℝn⊂ℂn\R^n \subset \C^n is not pluripolar. For n=1n = 1: if uu is subharmonic near a point of ℝ\R and u=−∞u = -\infty on an interval, then u≡−∞u \equiv -\infty near that point, since intervals are not polar. Applying this on complex lines parallel to the coordinate axes, one coordinate at a time, gives the general case. For n=1n = 1, pluripolar sets are exactly polar sets.

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Further reading

  1. Web https://en.wikipedia.org/wiki/Pluripolar_set
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