  Particles made of two fermions can in many cases be treated as elementary
bosons, but the conditions for this treatment to be valid are nontrivial. The
so-called "coboson formalism" is a powerful tool to tackle compositeness
effects relevant for instance for exciton physics and ultracold atomic dimers.
A key element of this theory is an ansatz for the ground state of N pairs,
built from the single-pair ground state combined with the exclusion principle.
We show that this ansatz can fail in one-dimensional systems which fulfill the
conditions expected to make the ansatz valid. Nevertheless, we also explain how
coboson theory can recover the correct ground state. Thus, our work highlights
limitations and strengths of the formalism and leads to a better treatment of
composite bosons.
