Regular matchstick graphs
  A graph G=(V,E) is called a unit-distance graph in the plane if there is an injective embedding of V in the plane such that every pair of adjacent vertices are at unit distance apart. If additionally the corresponding edges are non-crossing and all vertices have the same degree r we talk of a regular matchstick graph. Due to Euler's polyhedron formula we have $r\le 5$. The smallest known 4-regular matchstick graph is the so called Harborth graph consisting of 52 vertices. In this article we prove that no finite 5-regular matchstick graph exists and provide a lower bound for the number of vertices of 4-regular matchstick graphs. 
Sascha Kurz and Rom Pinchasi
math.CO