The matrix elements obey the triangle conditions \( J_{f}=J_{i} \) (\( \Delta J=0 \)). The Fermi operator has \( \pi _{O}=+1 \), and thus the initial and final nuclear states must have \( \pi _{i}\pi _{f}=+1 \) for the matrix element to be nonzero under the parity transform.

When isospin is conserved the Fermi matrix element must obey the isospin triangle condition \( T_{f}=T_{i} \) $(\Delta T=0)$, and the Fermi operator can only connect isobaric analogue states.