Some complexity problems on single input double output controllers

We investigate the time complexity of constructing single input double output state feedback controller structures, given the directed structure graph G of a system. Such a controller structure defines a restricted type of P₃-partition of the graph G. A necessary condition (*) is described and some classes of graphs are identified where the search problem of finding a feasible P₃-partition is polynomially solvable and, in addition, (*) is not only necessary but also sufficient for the existence of a P₃-partition. It is also proved that the decision problem on two particular graph classes - defined in terms of forbidden subgraphs - remains NP-complete, but is polynomially solvable on the intersection of those two classes. The polynomial-time solvability of some further related problems is shown, too.