= Nuclear mass parabola
{title2=$M(A,Z)\simeq a(A)Z^2+b(A)Z+c(A)$}
At fixed <mass number> $A$, the smooth <semi-empirical mass formula> makes the atomic <mass> approximately quadratic and convex in <atomic number> $Z$. Neighboring <nuclear isobars> undergo energetically allowed <beta decay> toward a discrete local minimum, subject to the electron-mass threshold for <beta-plus decay>. For odd $A$ there is one smooth branch; for even $A$, <nuclear pairing> splits the even-even and odd-odd branches. A local minimum against a single change of $Z$ need not be stable against <double beta decay>.
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