  In 1987, Michael Wiegner in his seminal paper [17] provided an important
result regarding the energy decay of Leray solutions $\boldsymbol u(\cdot,t)$
to the incompressible Navier-Stokes in $\mathbb{R}^{n}$: if the associated
Stokes flows had their $\hspace{-0.020cm}L^{2}\hspace{-0.050cm}$ norms bounded
by $O(1 + t)^{-\;\!\alpha} $ for some $ 0 < \alpha \leq (n+2)/4 $, then the
same would be true of $ \|\hspace{+0.020cm} \boldsymbol u(\cdot,t)
\hspace{+0.020cm} \|_{L^{2}(\mathbb{R}^{n})} $. The converse also holds, as
shown by Z.Skal\'ak [15] and by our analysis below, which uses a more
straightforward argument. As an application of these results, we discuss the
genericity problem of algebraic decay estimates for Leray solutions of the
unforced Navier-Stokes equations. In particular, we prove that Leray solutions
with algebraic decay generically satisfy two-sided bounds of the form
$(1+t)^{-\alpha}\lesssim \| \boldsymbol u(\cdot,t)\|_{L^2(\mathbb{R}^n)}
\lesssim (1+t)^{-\alpha}$.
