2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/182133We discuss the tightly bound (hydrino) solution of the Klein-Gordon equation for the Coulomb potential in 3 dimensions. We show that a similarly tightly bound state occurs for the Dirac equation in 2 dimensions. These states are unphysical since they disappear if the nuclear charge distribution is taken to have an arbitrarily small but non-zero radius.Submitted to Physics Letters AClassical PhysicsHigh Energy Physics - TheoryGeneral PhysicsQuantum Physicstext