Self-adjoint extensions for the Laplacian operator on non-compacts graphs and the existence and stability of standing waves for NLS models

Abstract: In this work we first study a systematic way to describe the unitary dynamics of the Schr\"odinger operator on a looping-edge graphs $\mathcal{G}$ (a graph consisting of a circle and a finite amount $N$ of infinite half-lines attached to a common vertex). We also apply our approach on $\mathcal T$-shaped graphs (metric graphs that consists of a pendant edge and a finite amount of half-lines attached to the common vertex). In both cases we obtain novel boundary conditions on the unique vertex that guarantee self-adjointness of the Laplacian operator and therefore unitary dynamics for the linear Schr\"odinger equation on both, looping-edge and $\mathcal{T}$-shaped graphs. In the second part, as an application of our abstract results, we discuss the existence and orbital (in)stability of some traveling waves for the cubic nonlinear Schr\"odiner equation posed on $\mathcal{G}$ with interactions of $\delta'$-type with strength $Z<0$ (continuity at the vertex is not required). We propose a dnoidal-profile in the circle with soliton tail-profiles on the half-lines and establish the existence and (in)stability depending on the relative size $N$ and $Z$.