Show that:
$$ \overline{\lim} n \{ n^2 \sqrt{2}\} = \infty $$
Sorry for being so succinct: $n \to \infty$ and $0 < \{ n^2 \sqrt{2} \} < 1$ so I am not sure which effect should win out.
Hint use Tauber theorem. That's not much of a hint but that's all I have to go on.
Checking the lim sup is not so difficult, but this does not constitute a proof:

For irrational positive $\alpha,$ the fractional part of $n^2 \alpha$ is equidistributed in the unit interval. This is Satz 13 in Weyl 1916.
In particular, it is larger than $1/2$ infinitely often.