= Stable small-signal inductance of a symmetric DC SQUID
{title2=$L=\Phi_0/[4\pi I_C|\cos(\pi\Phi/\Phi_0)|]$}
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 $|I|\ll2I_C|\cos(\pi\Phi/\Phi_0)|$, and fails near a half-integer flux quantum where the ideal critical current vanishes.
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