Kiselev, A. V., and Yu. Ershova. “Trace Formulae for Schrödinger Operators on Metric Graphs With Applications to Recovering Matching Conditions”. Methods of Functional Analysis and Topology, vol. 20, no. 2, June 2014, pp. 134-48, https://zen.imath.kiev.ua/index.php/mfat/article/view/574.