Nikolsky class
= Nikolsky class
{c}
{title2=$\mathcal N(\beta,L)$}
The $L^2$ Nikolsky class $\mathcal N(\beta,L)$ consists of functions with $r=\lfloor\beta\rfloor$ square-integrable derivatives whose highest derivative obeys the translation bound
$$
\lVert g^{(r)}(\mathord\cdot+t)-g^{(r)}\rVert_2
\leq L|t|^{\beta-r},
$$
with the corresponding finite-difference definition at integer smoothness.