The convergence of integral expressions for the effective properties of heterogeneous media

We present a very brief and general proof that the series expansion for the effective constant of a heterogeneous medium can be expressed in irreducible cluster integrals only. The argument does not use details of the cluster expansion. On the other hand, for any given expansion it provides a simple prescription to obtain irreducibility. For dielectric polarisation and other phenomena in which the interaction propagator falls off as R−3, this irreducibility guarantees absolute convergence of the integrals.