  While one of the most important and intriguing features of the topological
insulators is the presence of edge states, the closed-form expressions for the
edge states of some famous topological models are still lacking. Here, we focus
on the Kane-Mele model with and without Rashba spin-orbit coupling as a
well-known model to describe a two-dimensional version of the $\mathbb{Z}_2$
topological insulator to study the properties of its edge states analytically.
By considering the tight-binding model on a honeycomb lattice with zigzag
boundaries and introducing a perturbative approach, we derive explicit
expressions for the wave functions, energy dispersion relations, and the spin
rotations of the (generic) helical edge states. To this end, we first map the
edge states of the ribbon geometry into an effective two-leg ladder model with
momentum-dependent energy parameters. Then, we split the Hamiltonian of the
system into an unperturbed part and a perturbation. The unperturbed part has a
flat-band energy spectrum and can be solved exactly which allows us to consider
the remaining part of the Hamiltonian perturbatively. The resulting energy
dispersion relation within the first-order perturbation, surprisingly, is in
excellent agreement with the numerical spectra over a very wide range of
wavenumbers. Our perturbative framework also allows deriving an explicit form
for the rotation of the spins of the momentum edge states in the absence of
axial spin symmetry due to the Rashba spin-orbit interaction.
