It is fundamental to observe that there exists only one linear continuous endomorphism X on the space of tempered distributions such that X ( δ δ x ) = x δ δ x , for every real point x . It's possible to prove that the unique above endomorphism is necessarily defined by X ( ψ ψ ) = x ψ ψ , for every tempered distribution ψ ψ , where x denotes the coordinate function of the position line – defined from the real line into the complex plane by x : R → → C : x ↦ ↦ x .