An infinite family of adsorption models and restricted Lukasiewicz paths

We define $(k,\ell)$-restricted Lukasiewicz paths, $k\le\ell\in\mathbb{N}_0$, and use these paths as models of polymer adsorption. We write down a polynomial expression satisfied by the generating function for arbitrary values of $(k,\ell)$. The resulting polynomial is of degree $\ell+1$ and hence cannot be solved explicitly for sufficiently large $\ell$. We provide two different approaches to obtain the phase diagram. In addition to a more conventional analysis, we also develop a new mathematical characterization of the phase diagram in terms of the discriminant of the polynomial and a zero of its highest degree coefficient. We then give a bijection between $(k,\ell)$-restricted Lukasiewicz paths and "rise"-restricted Dyck paths, identifying another family of path models which share the same critical behaviour. For $(k,\ell)=(1,\infty)$ we provide a new bijection to Motzkin paths. We also consider the area-weighted generating function and show that it is a q-deformed algebraic function. We determine the generating function explicitly in particular cases of $(k,\ell)$-restricted Lukasiewicz paths, and for $(k,\ell)=(0,\infty)$ we provide a bijection to Dyck paths.