On the 3n+l Quantum Number in the Cluster Problem
  It has recently been suggested that an exactly solvable problem characterized by a new quantum number may underlie the electronic shell structure observed in the mass spectra of medium-sized sodium clusters. We investigate whether the conjectured quantum number 3n+l bears a similarity to the quantum numbers n+l and 2n+l, which characterize the hydrogen problem and the isotropic harmonic oscillator in three dimensions. 
Erik Koch (Max-Planck-Institut fuer Festkoerperforschung, Stuttgart)