  A constructive procedure is proposed for formulation of linear differential
equations invariant under global symmetry transformations forming a semi-simple
Lie algebra f. Under certain conditions f-invariant systems of differential
equations are shown to be associated with f-modules that are integrable with
respect to some parabolic subalgebra of f. The suggested construction is
motivated by the unfolded formulation of dynamical equations developed in the
higher spin gauge theory and provides a starting point for generalization to
the nonlinear case. It is applied to the conformal algebra o(M,2) to classify
all linear conformally invariant differential equations in Minkowski space.
Numerous examples of conformal equations are discussed from this perspective.
