Sequential diagnosability is co-NP complete

F.P. Preparata et al. (1967) introduced a graph-theoretical model for fault diagnosis called the PMC model. The question of determining the sequential diagnosability number of a system in the PMC model has remained open. The authors formalize the notion of sequential diagnosability. The question of determining the sequential diagnosability number of a system is addressed by showing that the appropriate decision problem is co-NP complete. The problem is also shown to be co-NP complete even when restricted to planar graphs both in the weighted and the BGM models