Oscillation criteria for even order neutral differential equations

Oscillation criteria are given for even order neutral type differential equations of the following form [x(t)+a(t)x(τ(t))](n)+ƒ(t,x(t),x(σ(t)))=0 , where f(t, x, y) ∈ C([0, ∞) × R2, R) and a, τ, σ ∈ C([0, ∞), R) such that 0 ≤ a(t) < 1, τ(t) < t, σ(t) ≤ t, and limt→∞ τ(t) = limt→∞σ(t) = ∞.