Right Shift Operator

**frater_cp** · June 2nd 2009, 06:32 AM

Let (e_n) be a total orthonormal sequence in a seperable Hilbert Space H and define the right shift operator to be the linear operator
T:H \longrightarrow H such that Te_n=e_{n+1} for n=1,2,\cdots.

Explain the name.
Find the range, null space, norm and Hilbert Adjoint operator of T.

**frater_cp** · June 12th 2009, 10:25 AM

Originally Posted by frater_cp

Let (e_n) be a total orthonormal sequence in a seperable Hilbert Space H and define the right shift operator to be the linear operator
T:H \longrightarrow H such that Te_n=e_{n+1} for n=1,2,\cdots.

Explain the name.
Find the range, null space, norm and Hilbert Adjoint operator of T.

can anyone help me solve this problem?

**siclar** · June 13th 2009, 04:09 PM

The name should be obvious in that the effect of the operator on each basis element is to shift the index "forward" by one. Example: $Te_1=e_2$ .

To find the range, you do the same as any other function. Take an element of the codomain (in this case H) and see if there is an element in the domain which is mapped to it. Example: Take $3e_4+e_7$ . Then an inspired guess tells us $T(3e_3+e_6)=3e_4+e_7$ , so we can conclude this is in the range. On the other hand, $e_1$ has no preimage.

For finding the nullspace, here is a hint: if an element $x=\displaystyle\sum_{i=1}^na_ie_i$ maps to zero, what does that imply about the $a_i$ . Use the linear independence of basis vectors!

For the norm, can you show this is an isometry?

For the adjoint, it suffices to find an operator A such that $\langle Tx,y\rangle=\langle x,Ay\rangle$ for all $x,y\in H$ . Can you show that the operator defined by $Ae_i=e_{i-1}$ if $i>1$ and $Ae_1=0$ works?

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