Degeneracy in Density Functional Theory: Topology in v- and n-Space
  This paper clarifies the topology of the mapping between v- and n-space in fermionic systems. Density manifolds corresponding to degeneracies g=1 and g>1 are shown to have the same mathematical measure: every density near a g-ensemble-v-representable (g-VR) n(r) is also g-VR (except ``boundary densities'' of lower measure). The role of symmetry and the connection between T=0 and T=0+ are discussed. A lattice model and the Be-series are used as illustrations. 
C. A. Ullrich and W. Kohn