Math 2.0

Several complex variables and analytic spaces Definition

Levi pseudoconvex domain

Builds on Levi form

Definition

Let Ω⊆ℂn\Omega \subseteq \C^n be a domain with C2C^2 boundary, and let ρ\rho be a defining function of Ω\Omega. For p∈∂Ωp \in \partial\Omega, let HpH_p be the complex tangent space of ∂Ω\partial\Omega at pp. The domain Ω\Omega is Levi pseudoconvex if the Levi form of ∂Ω\partial\Omega is positive semidefinite at every boundary point: ∑j,k=1n∂2ρ∂zj∂zˉk(p) wjwˉk≥0for all p∈∂Ω and all w∈Hp.\sum_{j,k=1}^{n} \frac{\partial^2 \rho}{\partial z_j \partial \bar z_k}(p)\, w_j \bar w_k \ge 0 \quad \text{for all } p \in \partial\Omega \text{ and all } w \in H_p.

Remarks

The condition does not depend on the choice of ρ\rho, because another defining function multiplies the Levi form by a positive number. Example: every convex domain with C2C^2 boundary is Levi pseudoconvex. Indeed, for a convex domain the real Hessian of ρ\rho at pp is positive semidefinite on the real tangent space of ∂Ω\partial\Omega at pp. For w∈Hpw \in H_p, both ww and iwi w lie in this tangent space, and Lρ(p;w)L_\rho(p; w) is one quarter of the sum of the real Hessian at ww and at iwi w. Non-example: the domain {z∈ℂ2:1<∣z1∣2+∣z2∣2<4}\set{z \in \C^2 : 1 < \abs{z_1}^2 + \abs{z_2}^2 < 4} fails at the inner sphere. There ρ(z)=1−∣z∣2\rho(z) = 1 - \abs{z}^2 is a defining function, and Lρ(p;w)=−∣w∣2<0L_\rho(p; w) = -\abs{w}^2 < 0 for w∈Hpw \in H_p, w≠0w \ne 0. When n=1n = 1 the space HpH_p is {0}\set{0}, so every planar domain with C2C^2 boundary is Levi pseudoconvex.

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Further reading

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