We study the average of sum, in the sense of integral.

Left shift operatorThroughout we consider the Hilbert space $L^2=L^2(\mathbb{R})$, the space of all complex-valued functions with real variable such that $f \in L^2$ if and only if \lVert f \rVert_2^2=\int_{-\infty}^{\infty}|f(t)|^2dm(t)

Analytic and quasi-analytic vectorsGuided by researches in function theory, operator theorists gave the analogue to quasi-analytic classes. Let $A$ be an operator in a Banach space $X$. $A$ is not necessarily bounded hence the domain $D(A)$ is not necessarily to be the whole space. We say $x \in X$ is a $C^\infty$ vector if $x \in \bigcap_{n \geq 1}D(A^n)$. This is quite intuitive if we consider the differential operator. A vector is analytic if the series \sum_{n=0}^{\infty}\lVert{A^n x}\rVert\frac{t^n}{n!}has a positive radius of convergence. Finally, we say $x$ is quasi-analytic for $A$ provided that \sum_{n=0}^{\infty}\left(\frac{1}{\lVert A^n x \rVert}\right)^{1/n} = \inftyor equivalently its nondecreasing majorant. Interestingly, if $A$ is symmetric, then $\lVert{A^nx}\rVert$ is log convex. Based on the density of quasi-analytic vectors, we have an interesting result. (Theorem) Let $A$ be a symmetric operator in a Hilbert space $\mathscr{H}$. If the set of quasi-analytic vectors spans a dense subset, then $A$ is essentially self-adjoint. This theorem can be considered as a corollary to the fundamental theorem of quasi-analytic classes, by applying suitable Banach space techniques in lieu.