Energy harvesting in a delay-induced parametric van der Pol–Duffing oscillator Zakaria Ghouli and Mohamed Belhaq Eur. Phys. J. Spec. Top., 230 18-20 (2021) 3591-3598 Published online: 29 July 2021 DOI: 10.1140/epjs/s11734-021-00243-5