Oscillation criteria for difference equations with unbounded delay

Consider the delay difference equation ξn+1−ξn+pnξτ(n)=0, N=0,1,2,…, where τ : N → Z is nondecreasing, τ(n) < n for n ε and limn → ∞ τ(n) = ∞, {pn} is a nonnegative sequence. Some oscillation criteria for this equation are obtained.