Optimizing Improved Hardy Inequalities

Let Omega be a bounded domain in R-N, N greater than or equal to 3, containing the origin. Motivated by a question of Brezis and Vazquez, we consider an Improved Hardy Inequality with best constant b, that we formally write as: -Delta greater than or equal to (N-2/2)(2)i/\x\(2) + bV(x). We first give necessary conditions on the potential V, under which the previous inequality can or cannot be further improved. We show that the best constant b is never achieved in H-0(1)(Omega), and in particular that the existence or not of further correction terms is not connected to the nonachievement of b in H-0(1)(Omega). Our analysis reveals that the original inequality can be repeatedly improved by adding on the right-hand side specific potentials. This leads to an infinite series expansion of Hardy's inequality. The series obtained is in some sense optimal. In establishing these results we derive various sharp improved Hardy-Sobolev Inequalities. (C) 2002 Elsevier Science (USA).