  Estimators $\Pi n$ for the correlation energy can be computed as roots of
effective characteristic polynomials of degree $n$. The coefficients of these
polynomials are derived from the terms of the perturbation series of the
energy. From a fourth-order M{\o}ller-Plesset (MP4) calculation one can
calculate with negligible effort a size-extensive estimator $\Pi 2$ that is in
many cases much closer to the full CI correlation energy of the ground state
than the MP4 value. [H.H.H. Homeier, J. Mol. Struct. (Theochem) 366, 161
(1996)] Here, we prove that the estimators $\Pi n$ for $n>2$ are size-extensive
if they are calculated from the MP series.
