Analytical approximations to nonlinear oscillation of nanoelectro-mechanical resonators G. M. Ismail, M. Abul-Ez, N. M. Farea and N. Saad Eur. Phys. J. Plus, 134 1 (2019) 47 Published online: 31 January 2019 DOI: 10.1140/epjp/i2019-12399-2