$J(x)$ is one of the Noether currents associated with global gauge transformations of flavour degrees of freedom, like a vector current $\bar{q}(x)\gamma^{\mu}q(x)$, or an axial–vector current $\bar{q}(x)\gamma^{\mu}\gamma_{5}q(x)$; but one could also consider gauge invariant local operators of gluon fields like ${\mbox{\rm tr}}G^{\mu\nu}(x)G_{\mu\nu}(x)$, or composite operators like $\bar{q}(x)\Gamma q(x)\bar{q}(x)\Gamma q(x)$. It has been shown by Källén and Lehmann [@Ka52; @Le54] quite a long time ago that two–point functions obey [*dispersion relations*]{}. The dispersion relation follows from the analyticity properties of $\Pi(q^2)$ as a complex function of $q^2$, the only energy–momentum invariant which appears in a two–point function. In full generality $\Pi(q^2)$ is an analytic function in the complex $q^2$–plane but for a cut in the real axis $0\le q^2 \le\infty$. It then follows that $$\label{eq:selfdisp}
\Pi(q^2)=\int_{0}^{\infty}dt\frac{1}{t-q^2
-i\epsilon}\frac{1}{\pi}{\mbox{\rm Im}}\Pi(t) + \mbox{\rm a} + \mbox{\rm b}q^2 +
\cdots,$$ where the degree of the arbitrary polynomial in the r.h.s. depends on the convergence properties of ${\mbox{\rm Im}}\Pi(t)$ when $t{\rightarrow}\infty$. The interest of this representation is that $\frac{1}{\pi}{\mbox{\rm Im}}\Pi(t)$ in the integrand, which is usually called the [*spectral function*]{}, is a physical cross–section. With $J(x)$ a current with specific quantum numbers, the spectral function is then directly related to the total cross section for the 