  We have found a (dense) basis for the N-representable, two-electron
densities, in which all N-representable two-electron densities can be expanded,
using positive coefficients. The inverse problem of finding a representative
wavefunction, giving a prescribed two-electron density, has also been solved.
The two-electron densities are found to lie in a convex set in a vector space.
We show that density matrices are more complicated objects than densities, and
density matrices do not seem to lie in a convex set. An algorithm to compute
the ground-state energy of a few-particle system is proposed, based on the
obtained results, where the correlation is treated exactly.
