# Domain of $A^{1/2}$ on $L^2(\mathbb{T}^2)$

Let $A$ be a densely defined unbounded self-adjoint operator defined on $L^2(\mathbb{T}^2)$, where $\mathbb{T}^2$ stands for the 2-torus. It is known that $A$ is positive, that is, $\langle Au, u\rangle \geq 0, u \in L^2(\mathbb{T}^2)$. It is also known that $\mathcal{D}(A) \subset H^1(\mathbb{T}^2)$. Let $A^{1/2}$ denote the square root of $A$, defined by the spectral theorem, which says that apriori, $A^{1/2}$ is a densely defined self-adjoint operator on $L^2(\mathbb{T}^2)$. But can we say something more, like, $\mathcal{D}(A^{1/2}) \subset H^{1/2}(\mathbb{T}^2)$?

Basically, I am trying to see how this generalizes to generic (at least compact) Riemannian manifolds, and trying to understand a relatively simple case first. Any reference would be appreciated. Thanks!

• Try the keyword "interpolation spaces." You will find your references. – Michael Renardy Apr 12 '15 at 0:31
• @MichaelRenardy This is what I understood: $\mathcal{D}(A^{1/2}) = [L^2, \mathcal{D}(A)]_{1/2} \subset [L^2, H^1]_{1/2} = H^{1/2}$. Could you please verify that I have properly understood this? Thanks. – anonymous Apr 12 '15 at 1:03