 Mean-field magnetic-susceptibility amplitude ratio

For a scalar quartic Landau free energy with $a=a_{0}t$ and positive quartic coefficient, the zero-field susceptibility is $1/(a_{0}t)$ above the transition and $1/(2a_0|t|)$ within a selected ordered phase below it. Their singular amplitudes have ratio two. Below the transition this is a one-sided pure-phase response: the globally minimizing magnetization jumps between opposite phases when the field crosses zero. The mean-field ratio is not asserted to be exact in every fluctuation-dominated system.