Synchronous Dynamical Systems on Directed Acyclic Graphs: Complexity and Algorithms.

Discrete dynamical systems serve as useful formal models to study diffusion phenomena in social networks. Several recent papers have studied the algorithmic and complexity aspects of some decision problems on synchronous Boolean networks, which are discrete dynamical systems whose underlying graphs are directed, and may contain directed cycles. Such problems can be regarded as reachability problems in the phase space of the corresponding dynamical system. Previous work has shown that some of these decision problems become efficiently solvable for systems on directed acyclic graphs (DAGs). Motivated by this line of work, we investigate a number of decision problems for dynamical systems whose underlying graphs are DAGs. We show that computational intractability (i.e., PSPACE -completeness) results for reachability problems hold even for dynamical systems on DAGs. We also identify some restricted versions of dynamical systems on DAGs for which reachability problem can be solved efficiently. In addition, we show that a decision problem (namely, Convergence), which is efficiently solvable for dynamical systems on DAGs, becomes PSPACE -complete for Quasi-DAGs (i.e., graphs that become DAGs by the removal of a single edge). In the process of establishing the above results, we also develop several structural properties of the phase spaces of dynamical systems on DAGs.