The Efimov effect is the remarkable theoretical observation that the number of bound states for three particles interacting via s-wave short range potentials may grow to infinity, as the pair interaction are just about to bind two particles.

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The Efimov effect is the remarkable theoretical
observation that the number of bound states for
three particles interacting via s-wave short
range potentials may grow to infinity, as the
pair interaction are just about to bind two
particles. Efimov states are loosely bound
and their wave functions extend far beyond those
of the the remaining two particles. There is
convincing theoretical evidence that the ground
state and exited state of the 4He trimer are
Efimov states. Efimov states can be
responsible for dramatic energy dependence of
scattering observables in three-body sector. An
interesting and challenging problem is to
determine what effects they would have in
resonant Bose-Einstein condensate. Reference
V. Efimov Phys. Lett. B33, 563 (1970).