Let , and let and denote the space of all infinitely differentiable functions with compact support on and on , respectively. The symbol denotes the dual space of .
define linear operators and from into , whereas and denote their adjoint operators.
If , and are locally integrable functions and every is the regular distribution corresponding to (i = 1, 2, 3) (this is written as ) and
almost everywhere on .
If satisfy equation (1), then they are of the form: , and , where , for some real and such that .