Identities on solutions of the wave equation in the enveloping algebra of the conformal group

The enveloping algebra of the conformal-group algebra of Minkowski space is regarded as an algebra of differential symmetry operators of the wave equation. It is shown that this algebra is graded. The structure of the enveloping algebra and of its ideal is investigated by means of the grading. The ideal consists of identities of elements of the enveloping algebra on solutions of the wave equation. All identities that consist of second-order operators are found.