= Isospin ladder strength in analogue beta decay
{title2=$|M_F|^2=T(T+1)-T_z(T_z\pm1)$}
The <Fermi transition> operator is an <isospin> ladder operator. Acting between normalized members of one <isospin multiplet> gives the displayed squared <matrix element>. An edge transition in a $T=3/2$ multiplet has Fermi strength three, whereas the transition between the two $T=1/2$ mirror members has strength one. This fixes the vector contribution; nonzero-spin analogue transitions can also have a <Gamow-Teller transition> contribution that is not fixed by this counting.
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