Oscillation of forced neutral differential equations with positive and negative coefficients

Some sufficient conditions are obtained for the oscillation of forced neutral differential equations with positive and negative coefficients [x(t)−R(t)x(t−r)]′+P(t)x(t−τ)−Q(t)x(t−σ)=f(t),t≥t0 where R(t),P(t),Q(t)∈C([t0,∞),R+),r>0,τ≥0,σ≥0R(t),P(t),Q(t)∈C([t0,∞),R+),r>0,τ≥0,σ≥0.