  We study binding energies in three isotopic chains ($^{100-130}$Sn,
$^{152-181}$Yb, and $^{181-202}$Pb) using the extended pairing model with
Nilsson single-particle energies. The exactly solvable nature of the model
means that the pairing strength G(A) required to reproduce the experimental
binding energies can be determined uniquely. The valence space consists of the
neutron single-particle levels between two closed shells corresponding to the
magic numbers 50-82 and 82-126. In all three isotopic chains, log(G(A)) has a
smooth quadratic behavior for even as well as odd nucleon numbers A; log(G(A))
for even and odd A are very similar. Remarkably, G(A) for all the Pb isotopes
can be also described by a two parameter expression that is inversely
proportional to the dimensionality of the model space.
