  We consider the Klein-Gordon equation in FRW-like spacetimes, with compact
space sections (not necessarily isotropic neither homogeneous). The bi-scalar
kernel allowing to select the positive-frequency part of any solution is
developed on mode solutions, using the eigenfunctions of the three-dimensional
Laplacian. Of course this kernel is not unique but, except (perhaps) when the
scale factor undergoes a special law of evolution, the metric has no more
symmetries (connected with the identity) than those inherited from the space
sections. As a result, all admissible definitions of the positive-frequency
kernel are related one to another by a unitary transformation which commutes
with the connected isometries of spacetime; any such kernel is invariant under
these isometries isometries. A physical interpretation is tentatively
suggested.
