The closed range theorem tells us that given two banach spaces X,Y,and a closed densely defined linear operator T：$X \to Y$. We have the following equivalence $R(T)$ is closed in $Y \iff R(T^{*})$ is closed in $X^{*} \iff R(T)=N(T^{*})^{\perp}$. This gives a complete characterisation, but I don't know whether it's convenient or not for application.

**Question 1:** Can anyone pose some examples to use this theorem ?

When we consider the case that $T$ is a bounded operator, it's true that if the range $TX$ has finite codimension in $Y$, then $TX$ is closed. So it seems that there are lots of bounded operators whose range is not closed. For instance, when $X=L^{p},1 \leq p<2$, $Y=L^{p'}$, where p' is the dual index. $T$ represents the fourier transform $\mathcal{F}$. Then

**Question 2:** Is $\mathcal{F}L^{p}$ closed in $L^{p'}$? I believe the range is not closed. BTW, I only know that the map is not surjective, if it is not closed, then we can see that actually the range is much smaller than $L^{p'}$