Comparison of Operator Topologies for Banach Spaces

functional-analysisgeneral-topologyoperator-theoryreal-analysis

Suppose $X$ and $Y$ are Banach spaces, and $L(X,Y)$ is the space of all bounded linear maps between $X$ and $Y$. I have seen in numerous places that in terms of $L(X,Y)$, the weak operator topology is weaker than the strong operator topology, and both of which are weaker than the norm topology.

I have seen this relationship proved in the context of Hilbert spaces (Relationship between different topologies of bounded operators on a Hilbert space or Comparison of Bounded Operator Topologies) or in terms of kernels (Strong Operator Topology vs Weak Operator Topology).

However, in Folland's text he states

These topologies are best understood in terms of convergence: $T_\alpha \rightarrow T$ strongly iff $T_\alpha x \rightarrow Tx$ in the norm topology of $Y$ for each $x \in X$, whereas $T_\alpha \rightarrow T$ weakly iff $T_\alpha x \rightarrow Tx$ in the weak topology of $Y$ for each $x \in X$. Thus the strong operator topology is stronger than the weak operator topology but weaker than the norm topology on $L(X,Y)$.

I fail to see how the above definitions of convergence imply the conclusion. Can anyone show the inclusion order of these topologies strictly in terms of convergence?

Best Answer

The topologies of function spaces are often described in terms of convergence. For example, the compact-open can be described in terms of uniform convergence whenever the underlying space has a uniform structure.

You should know about how weak convergence is so-called because it is much weaker than strong convergence: for example, in any separable Hilbert space, the orthonormal basis vectors weakly converge to $0$ but do not strongly tend to $0$. In $L^2$, the functions $\chi_{[n,n+1]}$ tend to zero weakly but not strongly.

$T_\alpha\to T$ in the SOT iff. for all $x\in X$ and all $\varepsilon\gt0$, there exists $A$ such that $\|(T_\alpha-T)x\|\lt\varepsilon$ for all $\alpha\gt A$. This can be seen by inspecting the basis. The convergence occurs in the WOT iff. for all $x,\varepsilon$ and for all continuous linear functionals $\psi$, $|\psi((T_\alpha-T)x)|\lt\varepsilon$. These are, fixing $x$, precisely the definitions of $T_\alpha x\to Tx$ strongly and $T_\alpha x\rightharpoonup Tx$ weakly; since strong implies weak but not the other way round, these are (in infinite spaces) strictly stronger and weaker conditions. Then if we assert that this should be true for all $x$ we get $T_\alpha\to T$ strongly and $T_\alpha\leftharpoonup T$ weakly as strictly stronger and weaker conditions, and the different topologies topologise the respective notions of convergence. However, the SOT is still “weak” in that the norm topology on $L(X,Y)$ is stronger: that topologises uniform convergence, since it requires that the maximum value of the difference $(T_\alpha-T)$ be bounded for all $x$, with the same bound.

I hope this interprets your question correctly. If you want a clear example, consider $X,Y=\ell^2$ and let $T_n((x_k)_{k\in\Bbb N})=x_n$, which is a continuous linear operator. $T_n\rightharpoonup0$ weakly, so, in the WOT, and also in the SOT, but not in the norm topology.

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