} = { \lim _{\varepsilon \searrow 0}}\frac{[\alpha - \beta ({ \rho ^\varepsilon _0})^0]{ \rho ^\varepsilon _0}}{{ \rho ^\varepsilon _0}} = - { \lim _{\varepsilon \searrow 0}}\frac{{ \varepsilon}A}{{ \rho ^\varepsilon _0}