Let $H^s(\mathbb T)$, where $s\in\mathbb R$, be the space of $2\pi$-periodic functions, $u(x)=\sum_{k\in\mathbb Z}\hat u_k\,\mathrm{e}^{ikx}$, such that $$ \|u\|_{H^s}^2=\sum_{k\in\mathbb Z}(1+k^2)^{s}\lvert \hat u_k\rvert^2<\infty. $$ Assume now that $s\in \big(\frac{1}{2},\frac{3}{2}\big)$. Is it true that there exist a constant $c=c_s$, such that $$ \lvert u(x)-u(y)\rvert\le c \|u\|_{H^s}\lvert x-y\rvert^{s-\frac{1}{2}}? $$ If $s=1$, then this is Morrey's inequality.

Any reference?

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