An L^p bound for the Riesz and Bessel potentials of orthonormal functions

Let ψ₁, ...,ψ_N be orthonormal functions in R^d and let u_i = (-Δ)^{- 1 2}ψ_i, or u_i = (-Δ + 1)^{- 1 2}ψ_i, and let p(x) = ∑|u_i(x)|². L^p bounds are proved for p, an example being ∥p∥_p ≤ A_dN ^{1 p} for d ≥ 3, with p = d(d - 2)^-1. The unusual feature of these bounds is that the orthogonality of the ψ_i, yields a factor N ^{1 p} instead of N, as would be the case without orthogonality. These bounds prove some conjectures of Battle and Federbush (a Phase Cell Cluster Expansion for Euclidean Field Theories, I, 1982, preprint) and of Conlon (Comm. Math. Phys., in press).