Pavel Exner and Stepan S. Manko Approximations of quantum-graph vertex couplings by singularly scaled potentials (216K, pdf) ABSTRACT. We investigate the limit properties of a family of Schr\"odinger operators of the form $H_ arepsilon= - rac{\mathrm{d}^2}{\mathrm{d}x^2}+ rac{\lambda( arepsilon)}{ arepsilon^2}Q ig( rac{x}{ arepsilon}ig)$ acting on $n$-edge star graphs with Kirchhoff conditions imposed at the vertex. The real-valued potential $Q$ is supposed to have compact support and $\lambda(