Universal singular free-energy amplitude ratio ($A_{+}/A_{-}$)

Under the scaling hypothesis for critical phenomena, $F_s=A_f|a_{t}t{|}^{D/y_t}{\Phi }_{\pm }(0)$ 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.