I desire to derive the values of the real permittivity from the imaginary permittivity ore vice versa using KK relations. It is known in KK equations that the integrands are defined in the bound [0,inf.] , but in practical applications, we do not have infinite frequencies and therefore the measured values will be different than the analytical values. So how can we apply the KK relations in a small frequency range [w1,w2], where w1>>0 and w2