Observational equivalence problem
This has led to various indirect approaches to observational semantics. As an extremal case, denotational semantics provides a model theoretic route to observational equivalence. There are also hybrid approaches that employ both denotational and operational techniques, such as Kripke logical relations [Sta85] and trace semantics [JR05]. Moreover, an operational and coinductive approach exists, under the name of applicative bisimilarity [Abr90].