An approximation to $\delta'$ couplings on graphs
  We discuss a general parametrization for vertices of quantum graphs and show, in particular, how the $\delta'_s$ and $\delta'$ coupling at an $n$ edge vertex can be approximated by means of $n+1$ couplings of the $\delta$ type provided the latter are properly scaled. 
Taksu Cheon, Pavel Exner