“…Classic papers include [62,13,48] on the square-root Klein-Gordon equation, [64,37,26,27] on the properties of the spectrum, stability of the matter [51,32,33,50], and eigenfunction decay [16]. More recent developments further addressed low-energy scattering theory [55], embedded eigenvalues and Neumann-Wigner type potentials [54], decay rates when magnetic potentials and spin are included [38], a relativistic Kato-inequality [39], Carleman estimates and unique continuation [56,31], or nonlinear relativistic Schrödinger equations [25,60,2]. Given its relationship with random processes with jumps, the V = 0 case has received much attention also in potential theory [57,36,15].…”