Rotating vector solving method applied for nonlinear oscillator L. Cveticanin, P. Suchy, I. Biro and M. Zukovic Eur. Phys. J. Spec. Top., 230 18-20 (2021) 3695-3703 Published online: 02 August 2021 DOI: 10.1140/epjs/s11734-021-00233-7