Laman graphs are known to be minimally rigid in the plane, but they might admit non-generic edge lengths such that there are infinitely many non-congruent vertex-injective realizations. In this talk we present some tools for determing families of such edge lengths. Our methods are based on NAC-colorings, whose existence characterizes the existence of edge lengths with infinitely many non-congruent realizations without the injectivity requirement. We exploit the fact that a subset of active NAC-colorings can be assigned to a particular motion of a graph. We can determine possible active AC-colorings if there are enough 4-cycle subgraphs. Moreover, certain active NAC-colorings impose some algebraic constraints on edge lengths. This allows to give an alternative proof of the result of Walter and Husty stating that the two motions of the complete bipartite graph on 3+3 vertices proposed by Dixon are the only possible ones. Next, we classify the motions of another Laman graph. This is a joint work with Georg Grasegger and Josef Schicho.

Date

Jun 12, 2019

Location

Lancaster, United Kingdom