Symmetry in the problem on wave-flow modes of a thin viscous-fluid layer

It is shown that a suitable transformation in the transverse-coordinate shift makes the set of equations describing the flow of viscous fluid films in the long-wavelength approximation invariant with respect to the reflection transformation. When constructing the flow models with the Galerkin spectral method, this symmetry in the equation for the longitudinal velocity stimulates its representation with the help of a set of functions symmetric with respect to the transverse coordinate.