Résumé en anglais

We prove that the best Lieb-Thirring constant when the eigenvalues of a Schrödinger operator −Δ+V(x) are raised to the power κ≥1 (κ≥3/2 in 1D and κ>1 in 2D) can never be attained for a potential having finitely many eigenvalues. We thereby disprove a conjecture of Lieb and Thirring in 2D that the best constant is given by the one-bound state case for 1<κ≲1.165. In a different but related direction, we also show that the cubic nonlinear Schrödinger equation admits no orthonormal ground state in 1D, for more than one function.