Multiplicity of bound states for a class of nonlinear Dirac equations on generalized star graphs with potentials

Abstract

In this paper we investigate the existence and multiplicity of bound states for a class of nonlinear Dirac equations on a generalized star graph \(\mathcal{G}\): \[ -ic\,σ_1\,\partial_xu+mc^2σ_3u+V(x)u=f(|u|)u,\qquad x\in\mathcal{G}, \] where \(V:\mathcal{G}\to\mathbb{R}\) is continuous, \(f\in C^1([0,\infty))\), \(m>0\), \(c>0\), and \(σ_1,σ_3\) are the Pauli matrices. Under suitable monotonicity and growth assumptions on \(f\), a uniform spectral gap and sufficiently long constant-potential intervals on \(k\) terminal edges yield at least \(2^k-1\) bound states, distinct up to a constant phase. The constant potentials on these intervals may differ. The construction uses half-line solutions and a uniform inverse estimate. For potentials tending to a constant at infinity, we prove compactness below the full-line ground level. In the flat-well setting, the solutions localized near individual terminal vertices lie below this level and provide a ground state through the generalized Nehari minimization. In particular, one localized solution is obtained for each minimum component containing a long terminal interval.

arXiv commentWe have revised and rewritten the manuscript based on the reviewers' comments. 36 pages, comments are welcome