Symmetry, phonons and rigid-layers modes in commensurate double wall carbon nanotubes E. Dobardžić, I. Milošević, T. Vuković, B. Nikolić and M. Damnjanović Eur. Phys. J. B, 34 4 (2003) 409-413 Published online: 09 September 2003 DOI: 10.1140/epjb/e2003-00238-8