  It is observed that one of Einstein-Friedmann's equations has formally the
aspect of a Sturm-Liouville problem, and that the cosmological constant,
$\Lambda$, plays thereby the role of spectral parameter (what hints to its
connection with the Casimir effect). The subsequent formulation of appropriate
boundary conditions leads to a set of admissible values for $\Lambda$,
considered as eigenvalues of the corresponding linear operator. Simplest
boundary conditions are assumed, namely that the eigenfunctions belong to $L^2$
space, with the result that, when all energy conditions are satisfied, they
yield a discrete spectrum for $\Lambda>0$ and a continuous one for $\Lambda<0$.
A very interesting situation is seen to occur when the discrete spectrum
contains only one point: then, there is the possibility to obtain appropriate
cosmological conditions without invoking the anthropic principle. This
possibility is shown to be realized in cyclic cosmological models, provided the
potential of the matter field is similar to the potential of the scalar field.
The dynamics of the universe in this case contains a sudden future singularity.
