Let $(e_n)$ be a total orthonormal sequence in a separable Hilbert space $H$ and define the right shift
operator to be the linear operator $T:H\rightarrow H$ such that $Te_n=e_{n+1}$, for $n=1, 2, \ldots.$
Find the range, null space, norm and Hilbert-adjoint operator of $T$.
[Math] Right Shift Operator
functional-analysis
Best Answer
To see the inclusion $\overline{\textrm{span}(e_2, e_3, \ldots)}\subset R(T)$, you do the following. You just check that $e_{k+1}\in R(T)$ for all $k\in\mathbb N$, and this is trivial because $e_{k+1}=Te_k$. So $\textrm{span}(e_2,e_3,\ldots)\in R(T)$.
Then it only remains to check that $R(T)$ is closed. This follows from the fact that $T$ is an isometry, i.e. $\|Tx\|=\|x\|$ for all $x\in H$. First, noting that $T^*$ is the operator that sends $e_1$ to $0$ and $e_{k+1}$ to $e_k$, $$ \|Tx\|^2=\|\sum_{k=1}^\infty\langle Tx,e_k\rangle\,e_k\|^2=\sum_{k=1}^\infty|\langle Tx,e_k\rangle|^2=\sum_{k=1}^\infty|\langle x,T^*e_k\rangle|^2=\sum_{k=2}^\infty|\langle x,e_{k-1}\rangle|^2=\sum_{k=1}^\infty|\langle x,e_k\rangle|^2=\|x\|^2, $$ so $T$ is isometric. Now, if $Tx_j\to y$, then $\{Tx_j\}$ is a Cauchy sequence; as $T$ is isometric $\{x_j\}$ is a Cauchy sequence too. Let $x=\lim x_j$. Then $$ y=\lim Tx_j=T(\lim x_j)=Tx\in R(T). $$ So $R(T)$ is closed.