Oscillation of Second-Order Delay Equations with Positive Feedback

We study oscillation of the second-order delay equation \[ x''(t)=x(τ(t)), \] with a nondecreasing delayed argument $τ(t)\le t$ satisfying $τ(t)\overset{t\to\infty}{\to}\infty$. Set \begin{equation*} a=\liminf_{t\to\infty}(t-τ(t)),\qquad b=\limsup_{t\to\infty}(t-τ(t)). \end{equation*} We describe