  We describe a class of exactly-solvable models of interacting bosons based on
the algebra SO(3,2). Each copy of the algebra represents a system of neutron
and proton bosons in a given bosonic level interacting via a pairing
interaction. The model that includes s and d bosons is a specific realization
of the IBM2, restricted to the transition regime between vibrational and
gamma-soft nuclei. By including additional copies of the algebra, we can
generate proton-neutron boson models involving other boson degrees of freedom,
while still maintaining exact solvability. In each of these models, we can
study not only the states of maximal symmetry, but also those of mixed
symmetry, albeit still in the vibrational to gamma-soft transition regime.
Furthermore, in each of these models we can study some features of F-spin
symmetry breaking. We report systematic calculations as a function of the
pairing strength for models based on s, d, and g bosons and on s, d, and f
bosons. The formalism of exactly-solvable models based on the SO(3,2) algebra
is not limited to systems of proton and neutron bosons, however, but can also
be applied to other scenarios that involve two species of interacting bosons.
