Feynman–Hellmann theorem for resonances and the quest for QCD exotica J. Ruiz de Elvira, U.-G. Meißner, A. Rusetsky and G. Schierholz Eur. Phys. J. C, 77 10 (2017) 659 Published online: 04 October 2017 DOI: 10.1140/epjc/s10052-017-5237-3