*(The question was originally posted on MSE.)*

**Preliminaries:** We know that the fractional-order Sobolev spaces $\mathrm{H}^s(\mathbb{R})$ and $\mathrm{H}^s(\mathbb{T})$ are closed under multiplication provided $s > 1/2$. This is proved for example in the following questions on MSE using the Fourier convolution theorems:

Thus we infer that $u^p \in \mathrm{H}^s(\mathbb{R})$ for all $p \in \mathbb{Z}_+$ whenever $u \in \mathrm{H}^s(\mathbb{R})$. Likewise for $\mathrm{H}^s(\mathbb{T})$.

**Question:** Is there a simple argument to conclude that $u^p \in \mathrm{H}^s(\mathbb{R})$ (and $\mathrm{H}^s(\mathbb{T})$) for all interpolating values $p \in [1, \infty)$, too?

More generally, from a MathOverflow question it is stated that the composition $f \circ u \in \mathrm{H}^s(\mathbb{R})$ for all real-valued $u \in \mathrm{H}^s(\mathbb{R})$ provided $f \in \mathrm{C}^{\lfloor s + 2 \rfloor}(\mathbb{R})$ with $f(0) = 0$. I would like to see references to this result or an outline of a proof.

**Revised question:** As pointed out by Joonas Ilmavirta below, I had made an elementary mistake concerning the expression $u^p$ for $p \in (1, \infty) \setminus \mathbb{Z}_+$, which should have been $|u|^p$ instead. Then I ask:

For which $p \in (1, \infty) \setminus \mathbb{Z}_+$ and $s > \frac{1}{2}$ is $|u|^p$ or $u|u|^{p - 1}$ in $\mathrm{H}^s(\mathbb{T})$ whenever $u \in \mathrm{H}^s(\mathbb{T})$?

(Similarly for $\mathrm{H}^s(\mathbb{R})$, but the periodic case is most important.)

Note: $|\cdot|^p$ and $\cdot |\cdot|^{p-1} \in \mathrm{C}^{\lfloor p \rfloor}$ for $p > 1$.