AbstractWe prove that the eigenvalues of the n-dimensional massive Dirac operator $${\mathscr {D}}_0 + V$$
D
0
+
V
, $$n\ge 2$$
n
≥
2
, perturbed by a potential V, possibly non-Hermitian, are contained in the union of two disjoint disks of the complex plane, provided V is sufficiently small with respect to the mixed norms $$L^1_{x_j} L^\infty _{{\widehat{x}}_j}$$
L
x
j
1
L
x
^
j
∞
, for $$j\in \{1,\dots ,n\}$$
j
∈
{
1
,
⋯
,
n
}
. In the massless case, we prove instead that the discrete spectrum is empty under the same smallness assumption on V, and in particular the spectrum coincides with the spectrum of the unperturbed operator: $$\sigma ({\mathscr {D}}_0+V)=\sigma ({\mathscr {D}}_0)={\mathbb {R}}$$
σ
(
D
0
+
V
)
=
σ
(
D
0
)
=
R
. The main tools used are an abstract version of the Birman–Schwinger principle, which allows in particular to control embedded eigenvalues, and suitable resolvent estimates for the Schrödinger operator.
Upper eigenvalue bounds for the Kirchhoff Laplacian on embedded metric graphs
