From local correlations to regional systematics
Local correlations of $2^+_1$ excitation energies and B(E2, $2^+_1\rightarrow \text{ g.s.}$) values require linear $N_pN_n$ systematics in a logarithmic scale, as confirmed by an experiment survey. Based on local correlations of $α$-decay energies, neutron separation energies, and proton separation energies, one can decouple them into their proton and neutron contributions separately. These contributions exhibit smooth regional systematics beyond the $N_pN_n$ scheme.
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