  It is shown that the ground-state energy as a functional solely of the
electron density is determined by the asymptotic value of the derivative of the
degree-one homogeneous extension of the universal density functional F[n] at
the given electron number. This has the consequence that its derivative cannot
be properly determined. Carrying out the derivative of E[n[N,v]] with respect
to v(r) leads to a paradox, which is resolved by the non-differentiability of
E[n] if one follows traditional wisdom regarding the non-invertibility of the
linear response function. However, considering the derivative of v[n[v]]
through the one-electron case shows that this paradox has a more elementary
origin, namely, an unaccounted restriction of the v(r) domain.
