The four-dimensional Martínez Alonso–Shabat equation: Differential coverings and recursion operators

We apply Cartan’s method of equivalence to find a contact integrable extension for the structure equations of the symmetry pseudo-group of the four-dimensional Martínez Alonso–Shabat equation. From this extension we derive two differential coverings including coverings with one and two non-removable parameters. Then we apply the same approach to the tangent covering and construct a recursion operator for symmetries of the equation under study.