On the algebras of nonlocal symmetries for the (modified) 4D Martı́nez Alonso-Shabat equation

We consider two four-dimensional Lax-integrable equations known as the 4D Martı́nez Alonso-Shabat equation and the modified Martı́nez Alonso-Shabat equation, respectively. We construct two differential coverings for both of them and describe the algebras of nonlocal symmetries in these coverings. We also analyze the actions of the known recursion operators on these nonlocal symmetries.
Partially based on a joint work with Joseph Krasil'shchik.