Symmetry Groupoids and Admissible Vector Fields for Coupled Cell Networks

The space of admissible vector fields, consistent with the structure of a network of coupled dynamical systems, can be specified in terms of the network's symmetry groupoid. The symmetry groupoid also determines the robust patterns of synchrony in the network --- those that arise because of the network topology. In particular, synchronous cells can be identified in a canonical manner to yield a quotient network. Admissible vector fields on the original network induce admissible vector fields on the quotient, and any dynamical state of such an induced vector field can be lifted to the original network, yielding an analogous state in which certain sets of cells are synchronized. We specify necessary and sufficient conditions for all admissible vector fields on the quotient to lift in this manner. These conditions are combinatorial in nature, and the proof uses invariant theory for the symmetric group. We also relate the symmetry groupoid of a quotient to that of the original network, and show that there is a close analogy with the usual normalizer symmetry that arises in group-equivariant dynamics.