A Hille-Wintner comparison theorem for second order differential systems Garret J. Etgen; Roger T. Lewis — 1980 Czechoslovak Mathematical Journal
The essential spectrum of relativistic multi-particle operators Roger T. Lewis; Heinz Siedentop; Simeon Vugalter — 1997 Annales de l'I.H.P. Physique théorique
A minimax principle for eigenvalues in spectral gaps: Dirac operators with Coulomb potentials. Griesemer, Marcel; Lewis, Roger T.; Siedentop, Heinz — 1999 Documenta Mathematica