2002/05/31 by Elliott H. Lieb, Robert Seiringer, Jakob Yngvason
Mathematics · Physics and Astronomy · #math.FA #math-ph #math.CA #math.MP
published as Ann. of Math. (2), Vol. 158 (2003), no. 3, 1067--1080 · 14 pages published version
arxiv created 2004/03/26 · arxiv updated 2009/11/30
The classic Poincare inequality bounds the Lq-norm of a function f in a bounded domain Ω⊂ \Rn in terms of some Lp-norm of its gradient in Ω. We generalize this in two ways: In the first generalization we remove a set Γ from Ω and concentrate our attention on Λ= Ω∖ Γ. This new domain might not even be connected and hence no Poincare inequality can generally hold for it, or if it does hold it might have a very bad constant. This is so even if the volume of Γ is arbitrarily small. A Poincare inequality does hold, however, if one makes the additional assumption that f has a finite Lp gradient norm on the whole of Ω, not just on Λ. The important point is that the Poincare inequality thus obtained bounds the Lq-norm of f in terms of the Lp gradient norm on Λ (not Ω) plus an additional term that goes to zero as the volume of Γ goes to zero. This error term depends on Γ only through its volume. Apart from this additive error term, the constant in the inequality remains that of the `nice' domain Ω. In the second generalization we are given a vector field A and replace ∇ by ∇ +i A(x) (geometrically, a connection on a U(1) bundle). Unlike the A=0 case, the infimum of ‖(∇ +i A)f‖p over all f with a given ‖f‖q is in general not zero. This permits an improvement of the inequality by the addition of a term whose sharp value we derive. We describe some open problems that arise from these generalizations.