Periodic Sobolev space
= Periodic Sobolev space
{title2=$H^s(\mathbb T^d)$}
The periodic Sobolev space $H^s(\mathbb T^d)$ consists of periodic distributions whose Fourier coefficients obey
$$
\sum_{n\in\mathbb Z^d}(1+|n|^2)^s|\widehat u_n|^2<\infty.
$$
In one dimension, $H^1(\mathbb T)$ embeds continuously into $L^\infty(\mathbb T)$ and is a Banach algebra.