  In the formation control problem for autonomous robots a distributed control
law steers the robots to the desired target formation. A local stability result
of the target formation can be derived by methods of linearization and center
manifold theory or via a Lyapunov-based approach. It is well known that there
are various other undesired invariant sets of the robots' closed-loop dynamics.
This paper addresses a global stability analysis by a differential geometric
approach considering invariant manifolds and their local stability properties.
The theoretical results are then applied to the well-known example of a cyclic
triangular formation and result in instability of all invariant sets other than
the target formation.
