@Article{ 
  MetPal0a,
  author = { Metafune, Giorgio and Pallara, Diego },
  title = { Discreteness of the spectrum for a class of differential operators with unbounded coefficients in ${\mathbb R}^n$ },
  journal = { Rend. Mat. Acc. Lincei },
  year = { 2000 },
  volume = { 11 },
  pages = { 9-19 },
  URL = { http://cvgmt.sns.it/paper/2427/ },
  note = { cvgmt preprint}
}