For all $k \in \mathbb{N}^*$, we set $g_k(x) = \sqrt{2}\sin(k\pi x)$. We denote by $G = \operatorname{Vect}\left((g_k)_{k \in \mathbb{N}^*}\right)$ and $H = G^\perp$.
Justify that, for all $(f,g) \in E^2$, we have
$$\langle T(f), g \rangle = \langle f, T(g) \rangle$$
One may use question 12.