Application of differential forms to construction of nonlocal symmetries

Differential forms are used for construction of nonlocal symmetries of partial differential equations with conservation laws. Every conservation law allows to introduce a nonlocal variable corresponding to the conserved quantity. A prolongation technique is offered for action of symmetry operators on these nonlocal variables. It is shown how to introduce these variables for the symmetry group to be the same. A new hidden symmetry and corresponding group-invariant solution are found for gas dynamics equations.