  This paper examines stability properties of distance-based formation control
when the underlying topology consists of a rigid graph and a flex node
addition. It is shown that the desired equilibrium set is locally
asymptotically stable but there exist undesired equilibria. Specifically, we
further consider two cases where the rigid graph is a triangle in 2-D and a
tetrahedral in 3-D, and prove that any undesired equilibrium point in these
cases is unstable. Thus in these cases, the desired formations are almost
globally asymptotically stable.
