Perspectives on the theory of tri-neutron and tetra-neutron systems

Theoretical efforts to investigate tri-neutron (3n) and tetra-neutron (4n) structures range from ab initio to phenomenological. I will review these approaches and their results along with their quantified uncertainties, where available, with a particular emphasis on ab initio approaches. Key issues include, but are not limited to, the limits of the underlying assumptions made (methods, inter-nucleon interactions) and the estimated convergence of theoretical calculations. I will also present some speculations on 4n structures at excitation energies above the recently reported experimental 4n structure.