nonmodelcheckable

Nonmodelcheckable is an informal designation used in verification and formal methods to describe system properties or verification tasks that cannot be fully or decidably verified by conventional model checking techniques. The term highlights limitations arising from expressiveness restrictions of specification languages, undecidability in underlying models, or practical barriers such as state-space explosion and unbounded resources.

Instances commonly called nonmodelcheckable include properties that require reasoning about unbounded data structures, arbitrary numeric ranges, dynamic topologies, or intricate fairness and liveness conditions that are not captured by the temporal logics supported by a given model checker. Some verification goals are undecidable in the chosen computational model (for example, general properties of Turing-complete programs), and others are simply not expressible in a particular logic (for example, certain hyperproperties or quantitative constraints).

Addressing nonmodelcheckable tasks typically involves complementary techniques: abstraction and refinement, parameterized or symbolic verification, theorem proving, runtime verification, and statistical or probabilistic methods. Engineers often restrict models, add invariants, or apply cutoff theorems to make instances amenable to model checking. Where exact verification is impossible, semi-decision procedures and best-effort analyses provide partial assurance.

The concept underscores practical and theoretical boundaries of automated verification and guides the selection of methods for different classes of systems and properties. It is not a formal classification but a useful label in discussions of verification feasibility and tool selection.