“…[11,41,42,43,44,45,54,62] for a non-exhaustive list, but also other types of operators such as first order operators [13,66,69], operators related to diffusion processes on metric graphs [27,28,36,67], as well as higher-order operators have been investigated as well, e.g. in [4,7,20,21,50,55,68]. Furthermore, descriptions of certain non-self-adjoint operators on metric graphs appear in the literature [26].…”