Symmetric Versions of Laman's Theorem

Recent work has shown that if an isostatic bar and joint framework possesses non-trivial symmetries, then it must satisfy some very simply stated restrictions on the number of joints and bars that are ‘fixed’ by various symmetry operations of the framework. For the group C3 which describes 3-fold rotational symmetry in the plane, we verify the conjecture proposed in [4] that these restrictions on the number of fixed structural components, together with the Laman conditions, are also sufficient for a framework with C3 symmetry to be isostatic, provided that its joints are positioned as generically as possible subject to the given symmetry constraints. In addition, we establish symmetric versions of Henneberg’s Theorem and Crapo’s Theorem for C3 which provide alternate characterizations of ‘generically’ isostatic graphs with C3 symmetry. As shown in [19], our techniques can be extended to establish analogous results for the symmetry groups C2 and Cs which are generated by a half-turn and a reflection in the plane, respectively.