  We investigate the energy spectrum of single layer black phosphorene
nanoribbons (BPN) by means of a low-energy expansion of a recently proposed
tight-binding model that describes electron and hole bands close to the Fermi
energy level. Using the continuum approach, we propose boundary conditions
based on sublattice symmetries for BPN with zigzag and armchair edges and show
that our results for the energy spectra exhibit good agreement with those
obtained by using the five-parameter tight-binding model. We also explore the
behaviour of the energy gap versus the nanoribbon width $W$. Our findings
demonstrate that band gap of armchair BPNs scale as $1/W^2$, while zigzag BPNs
exhibit a $1/W$ tendency. We analyse the different possible combinations of the
zigzag edges that result two-fold degenerate and non-degenerate edge states.
Furthermore, we obtain expressions for the wave functions and discuss the limit
of validity of such analytical model.
