  Computation Tree Logic (CTL) is one of the central formalisms in formal
verification. As a specification language, it is used to express a property
that the system at hand is expected to satisfy. From both the verification and
the system design points of view, some information content of such property
might become irrelevant for the system due to various reasons, e.g., it might
become obsolete by time, or perhaps infeasible due to practical difficulties.
Then, the problem arises on how to subtract such piece of information without
altering the relevant system behaviour or violating the existing specifications
over a given signature. Moreover, in such a scenario, two crucial notions are
informative: the strongest necessary condition (SNC) and the weakest sufficient
condition (WSC) of a given property. To address such a scenario in a principled
way, we introduce a forgetting-based approach in CTL and show that it can be
used to compute SNC and WSC of a property under a given model and over a given
signature. We study its theoretical properties and also show that our notion of
forgetting satisfies existing essential postulates of knowledge forgetting.
Furthermore, we analyse the computational complexity of some basic reasoning
tasks for the fragment CTL_AF in particular.
