Comparison results for oscillations of delay equations

We established a comparison result for the oscillation of all solutions of the linear delay equation with positive and negative coefficients $$\dot x(t) + P(t)x(t - \tau ) - Q(t)x(t - \sigma ) = 0,t \geqslant t_0$$ in terms of the oscillation of the « limiting » equation $$\dot y(t) + py(t - \tau ) - qy(t - \sigma ) = 0$$ (*) where p= lim infP(t) and q=lim supQ(t). t→∞ t→∞