Stable small-signal inductance of a symmetric DC SQUID ($L={\Phi }_0/[4\pi I_C|\cos (\pi \Phi /{\Phi }_0)|]$)

For an ideal symmetric two-junction interferometer with negligible self-flux, the stable zero-current phase is zero or pi according to the sign of the flux cosine. Linearization about that stable phase gives positive inductance. The stronger small-current condition is $|I|\ll 2I_C|\cos (\pi \Phi /{\Phi }_0)|$, and fails near a half-integer flux quantum where the ideal critical current vanishes.