Rigorous perturbation theory versus variational methods in the spectral   study of carbon nanotubes
  Recent two-photon photo-luminescence experiments give accurate data for the ground and first excited excitonic energies at different nanotube radii. In this paper we compare the analytic approximations proved in \cite{CDR}, with a standard variational approach. We show an excellent agreement at sufficiently small radii. 
Horia D. Cornean (MATH. Dept., Aalborg University), Thomas G. Pedersen   (Physics and Nanotech. Dept., Aalborg University), ...