Discrimination by parallel observers

The main result of the paper is a proof of the following equivalence: two pure lambda terms are observationally equivalent in the lazy concurrent lambda calculus if they have the same Levy-Longo trees. It follows that contextual equivalence coincides with behavioural equivalence (bisimulation) as considered by Sangiorgi (1994). Another consequence is that the discriminating power of concurrent lambda contexts is the same as that of Boudol-Laneve's contexts with multiplicities (1996).