For a two-level problem in one dimension, an implicit relation is derived relating the kinetic energy density t s (x) to the electron density p(x) plus its first and second derivatives.As an example, tle harmonic oscillator potential is used to make this relation explicit.
From the explicit solution of a two-level problem previously obtained, it is proved that the single-particle kinetic energy density functional ts[\ensuremath{\rho}] is the density \ensuremath{\rho} times a function solely of the ratio \ensuremath{\rho}'/\ensuremath{\rho}, with \ensuremath{\rho}' the first derivative of \ensuremath{\rho}. For an n-level system with a harmonic-oscillator potential, the form of ts[\ensuremath{\rho}] is explicitly obtained. Again ts[\ensuremath{\rho}] involves only \ensuremath{\rho} and \ensuremath{\rho}'; no higher derivatives appear.
Using the known perturbation series for the idempotent Dirac density matrix in powers of a given one-body potential V(r), Stoddart and March (SM) generated a corresponding series for the kinetic energy density. While the general term of the SM series is known, a summation has not been achieved to date. A contribution to solve this problem is made here by exhibiting an explicit form of the above kinetic energy density for Fermions filling an arbitrary number of closed shells, when the confinement is harmonic. This example is of considerable current interest because of ongoing experiments on ultracold atomic gases of Fermions.