Inequalities for means of chords, with application to isoperimetric problems

We consider a pair of isoperimetric problems arising in physics. The first concerns a Schrödinger operator in L2(R2) with an attractive interaction supported on a closed curve Γ, formally given by −∆ − αδ(x−Γ); we ask which curve of a given length maximizes the ground state energy. In the second problem we have a loop-shaped thread Γ in R3, homogeneously charged but not conducting, and we ask about the (renormalized) potential-energy minimizer. Both problems reduce to purely geometric questions about inequalities for mean values of chords of Γ. We prove an isoperimetric theorem for p-means of chords of curves when p ≤ 2, which implies in particular that the global extrema for the physical problems are always attained when Γ is a circle. The article finishes with a discussion of the p–means of chords when p > 2.