In recent years, algebras and modules of differential operators have been
extensively studied. Equivariant quantization and dequantization establish a
tight link between invariant operators connecting modules of differential
operators on tensor densities, and module morphisms that connect the
corresponding dequantized spaces. In this paper, we investigate dequantized
differential operators as modules over a Lie subalgebra of vector fields that
preserve an additional structure. More precisely, we take an interest in
invariant operators between dequantized spaces, viewed as modules over the Lie
subalgebra of infinitesimal contact or projective contact transformations. The
principal symbols of these invariant operators are invariant tensor fields. We
first provide full description of the algebras of such affine-contact- and
contact-invariant tensor fields. These characterizations allow showing that the
algebra of projective-contact-invariant operators between dequantized spaces
implemented by the same density weight, is generated by the vertical cotangent
lift of the contact form and a generalized contact Hamiltonian. As an
application, we prove a second key-result, which asserts that the Casimir
operator of the Lie algebra of infinitesimal projective contact
transformations, is diagonal. Eventually, this upshot entails that invariant
operators between spaces induced by different density weights, are made up by a
small number of building bricks that force the parameters of the source and
target spaces to verify Diophantine-type equations.Comment: 22 page
