  We give an algorithm to write down all conformally invariant differential
operators acting between scalar functions on Minkowski space. All operators of
order k are nonlinear and are functions on a finite family of functionally
independent invariant operators of order up to k. The independent differential
operators of second order are three and we give an explicit realization of
them. The applied technique is based on the jet bundle formalism,
algebraization of the the differential operators, group action and dimensional
reduction. As an illustration of this method we consider the simpler case of
differential operators between analytic functions invariant under the modular
group. We give a power series generating explicitly all the functionally
independent invariant operators of an arbitrary order.
