“…These objects yield exactly solvable models [2,4,5,6,8,11,12,26,38,45] and have been widely used in applications in quantum mechanics (e.g. in models of low-energy scattering [3,13,14,35] and quantum systems with boundaries [22,23,24,27,32]), condensed matter physics [10,17,25] and, more recently, on the approximation of thin quantum waveguides by quantum graphs [1,15,16,20].…”