Topological invariant quintessence
  The issues of quintessence and cosmic acceleration can be discussed in the
framework of $F(R, {\cal G})$ theories of gravity where $R$ is the Ricci
curvature scalar and ${\cal G}$ is the Gauss-Bonnet topological invariant. It
is possible to show that such an approach exhausts all the curvature content
related to the Riemann tensor giving rise to a fully geometric approach to dark
energy.
