Consider a Langevin process, that is an integrated Brownian motion,
constrained to stay on the nonnegative half-line by a partially elastic
boundary at 0. If the elasticity coefficient of the boundary is greater than or
equal to a critical value (0.16), bounces will not accumulate in a finite time
when the process starts from the origin with strictly positive velocity. We
will show that there exists then a unique entrance law from the boundary with
zero velocity, despite the immediate accumulation of bounces. This result of
uniqueness is in sharp contrast with the literature on deterministic second
order reflection. Our approach uses certain properties of real-valued random
walks and a notion of spatial stationarity which may be of independent
interest.Comment: 30 pages, 1 figure. In this new version, the introduction and the
preliminaries in particular have been rewritten (for a dramatic change
