# Fractional powers of positive self-adjoint operators

Consider two positive unbounded operators $A$ and $B$ densely defined on a Hilbert space $H$ self-adjoint on a domain $\mathcal{D}(A) = \mathcal{D}(B) = H_1$. By the spectral theorem, we can define the fractional powers of $A$ and $B$ as self-adjoint linear operators on $H$. My question is, is $\mathcal{D}(A^{\alpha}) = \mathcal{D}(B^\alpha)$, where $\alpha \in (0, 1)$ necessarily? Is this part of the spectral theorem? If yes, where can I find such a statement?

• Hmm seems not expectable but interesting... – C-Star-W-Star Jun 17 '15 at 3:12

If $A$ and $B$ are positive definite densely-defined selfadjoint linear operators on a complex Hilbert space $H$, then $AB^{-1}$ and $BA^{-1}$ are bounded if $\mathcal{D}(A)=\mathcal{D}(B)$. Then $A^{s}B^{-s}$ and $B^{s}A^{-s}$ are bounded for $0 < s \le 1$ with $$\|A^{s}B^{-s}\| \le \|AB^{-1}\|^{s} \\ \|B^{s}A^{-s}\| \le \|BA^{-1}\|^{s}.$$ This implies that $\mathcal{D}(A^{s})=\mathcal{D}(B^{s})$ for $0 < s \le 1$.
• How do you deduce boundedness from positive definite? I mean one has $\mathcal{W}(H)>0$ but only $\sigma(H)=\overline{\mathcal{W}(H)}$ for $H=H^*$. – C-Star-W-Star Jun 22 '15 at 15:41
• @Freeze_S : positive definite means $A \ge \delta I$ with $\delta > 0$, which gives boundedness of $A^{-1}$. – DisintegratingByParts Jun 22 '15 at 16:03