= Universal singular free-energy amplitude ratio
{title2=$A_+/A_-$}
Under the <scaling hypothesis for critical phenomena>, $F_s=A_f|a_tt|^{D/y_t}\Phi_\pm(0)$ at zero field. The common free-energy and thermal metric factors cancel from $A_+/A_-$, leaving a property of the <universality class>. This concerns the singular part at equal $|t|$, not the ratio of full <free energies> including arbitrary analytic identity terms. Matching constants in $\Phi_\pm$ belong to the same scaling solution and cannot be assigned independently.
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