Bound states of nonlinear Dirac equation on noncompact quantum graphs with localized nonlinearities

This paper investigates the nonlinear Dirac equation (NLDE) on noncompact quantum graphs featuring localized nonlinearities, specifically under Kirchhoff-type vertex conditions. Our primary focus is on the existence and multiplicity of bound states, which emerge as critical points of the NLDE Lagrangian functional. The associated action functional is strongly indefinite, and notably, the Palais-Smale condition fails to hold. To address these challenges, we employ recently developed critical point theorems to derive our results.