Investigation of surface effects on the natural frequency of a functionally graded cylindrical nanoshell based on nonlocal strain gradient theoryKhashayar Ghorbani, Ali Rajabpour, Majid Ghadiri and Zahra KeshtkarEur. Phys. J. Plus, 135 9 (2020) 701DOI: https://doi.org/10.1140/epjp/s13360-020-00712-1