Functional calculus: Does $A$ commute with $e^{iA^2}$? Let $A$ be an unbounded self-adjoint operator. Is it then true that $A$ commutes with $e^{iA^2}$? This sounds natural to me but I have no clue whether this is true in general. The problem is that we only did such a functional calculus for bounded(!) operators, so I don't even know whether something similarly exists for unbounded ones?
 A: The Spectral Theorem for $A$ is given in terms of a Borel Spectral measure $E$
$$
                  Ax = \int_{-\infty}^{\infty}\lambda dE(\lambda)x,
$$
and $x \in \mathcal{D}(A)$ iff
$$
            \int_{-\infty}^{\infty}\lambda^2 d\|E(\lambda)x\|^2 < \infty.
$$
The operator $e^{iA^2}$ is defined through the functional calculus as
$$
     e^{iA^2}x = \int_{-\infty}^{\infty}e^{i\lambda^2}dE(\lambda)x.
$$
This operator is unitary. For bounded Borel functions $f$, $g$, the operators $f(A)$ and $g(A)$ are bounded operators that satisfy
$$
       \int f(\lambda)dE(\lambda) \int g(\lambda) dE(\lambda)x = \int (fg)(\lambda)dE(\lambda)x,\;\;\; x\in\mathcal{H}.
$$
Therefore,
$$
            \int_{-R}^{R}\lambda dE(\lambda) e^{iA^2}x = \int_{-R}^{R}\lambda e^{iA^2}dE(\lambda)x  =e^{iA^2}\int_{-R}^{R}\lambda dE(\lambda)x \\
             \int_{-R}^{R}\lambda^2 d\|E(\lambda)e^{iA^2}x\|^2 =
\int_{-R}^{R}\lambda^2 d\|E(\lambda)x\|^2.
$$
As $R\rightarrow\infty$, the conclusion from the last integral is that $x\in\mathcal{D}(A)$ iff $e^{iA^2}x\in\mathcal{D}(A)$. Furthermore, using the first vector identity,
$$
          Ae^{iA^2}x = e^{iA^2}Ax,\;\;\; x\in\mathcal{D}(A).
$$
