Dynamics of Inflation and Dark Energy from $F(R, \mathcal{G})$ Gravity
  In this work we study certain classes of $F(R,{\cal G})$ gravity which have
appealing phenomenological features, with respect to the successful realization
of the dark energy and of the inflationary era. Particularly, we discuss the
general formalism and we demonstrate how several inflationary and dark energy
evolutions can be described in the context of $F(R,{\cal G})$ gravity. Also we
propose a unified model, in the context of which the early and late-time
dynamics are controlled by the $F(R,{\cal G})$ gravity, thus producing
inflation and the dark energy era, while the intermediate era is approximately
identical with standard Einstein-Hilbert gravity. Also we calculate the power
spectrum of the primordial curvature perturbations corresponding to the unified
$F(R,{\cal G})$ gravity model we propose, which as we demonstrate is nearly
scale invariant and compatible with the latest observational data constraints.
