  Relative binding energies (RBEs) within three isotopic chains
($^{100-130}$Sn, $^{152-181}$Yb, and $^{181-202 }$Pb) have been studied using
the exactly solvable extended pairing model (EPM) \cite{FPan04}. The unique
pairing strength $G$, which reproduces the experimental RBEs, has been
determined. Within EPM, $\log(G)$ is a smooth function of the model space
dimension $\dim(A)$, as expected for an effective coupling strength. In
particular, for the Pb and Sn isotopes $G$ can be described by a two parameter
expression that is inversely proportional to the dimensionality of the model
space, $G=\alpha\dim(A)^{-\beta}$ with $\beta\approx 1$. PACS Classification:
21.10.Dr Binding energies, 71.10.Li Pairing interactions in model systems, and
21.60.Cs Shell model.
