Weak operator topology

In functional analysis, the weak operator topology, often abbreviated WOT,^[1] is the weakest topology on the set of bounded operators on a Hilbert space $H$ , such that the functional sending an operator $T$ to the complex number $\langle Tx,y\rangle$ is continuous for any vectors $x$ and $y$ in the Hilbert space.

Explicitly, for an operator $T$ there is base of neighborhoods of the following type: choose a finite number of vectors $x_{i}$ , continuous functionals $y_{i}$ , and positive real constants $\varepsilon _{i}$ indexed by the same finite set $I$ . An operator $S$ lies in the neighborhood if and only if $|y_{i}(T(x_{i})-S(x_{i}))|<\varepsilon _{i}$ for all $i\in I$ .

Equivalently, a net $T_{i}\subseteq B(H)$ of bounded operators converges to $T\in B(H)$ in WOT if for all $y\in H^{*}$ and $x\in H$ , the net $y(T_{i}x)$ converges to $y(Tx)$ .

Strong operator topology

The WOT is the weakest among all common topologies on $B(H)$ , the bounded operators on a Hilbert space $H$ .

The strong operator topology, or SOT, on $B(H)$ is the topology of pointwise convergence. Because the inner product is a continuous function, the SOT is stronger than WOT. The following example shows that this inclusion is strict. Let $H=\ell ^{2}(\mathbb {N} )$ and consider the sequence $\{T^{n}\}$ of right shifts. An application of Cauchy-Schwarz shows that $T^{n}\to 0$ in WOT. But clearly $T^{n}$ does not converge to $0$ in SOT.

The linear functionals on the set of bounded operators on a Hilbert space that are continuous in the strong operator topology are precisely those that are continuous in the WOT (actually, the WOT is the weakest operator topology that leaves continuous all strongly continuous linear functionals on the set $B(H)$ of bounded operators on the Hilbert space H). Because of this fact, the closure of a convex set of operators in the WOT is the same as the closure of that set in the SOT.

It follows from the polarization identity that a net $\{T_{\alpha }\}$ converges to $0$ in SOT if and only if $T_{\alpha }^{*}T_{\alpha }\to 0$ in WOT.

Weak-star operator topology

The predual of B(H) is the trace class operators C₁(H), and it generates the w*-topology on B(H), called the weak-star operator topology or σ-weak topology. The weak-operator and σ-weak topologies agree on norm-bounded sets in B(H).

A net {T_α} ⊂ B(H) converges to T in WOT if and only Tr(T_αF) converges to Tr(TF) for all finite-rank operator F. Since every finite-rank operator is trace-class, this implies that WOT is weaker than the σ-weak topology. To see why the claim is true, recall that every finite-rank operator F is a finite sum

F=\sum _{i=1}^{n}\lambda _{i}u_{i}v_{i}^{*}.

So {T_α} converges to T in WOT means

{\text{Tr}}\left(T_{\alpha }F\right)=\sum _{i=1}^{n}\lambda _{i}v_{i}^{*}\left(T_{\alpha }u_{i}\right)\longrightarrow \sum _{i=1}^{n}\lambda _{i}v_{i}^{*}\left(Tu_{i}\right)={\text{Tr}}(TF).

Extending slightly, one can say that the weak-operator and σ-weak topologies agree on norm-bounded sets in B(H): Every trace-class operator is of the form

S=\sum _{i}\lambda _{i}u_{i}v_{i}^{*},

where the series $\sum \nolimits _{i}\lambda _{i}$ converges. Suppose $\sup \nolimits _{\alpha }\|T_{\alpha }\|=k<\infty ,$ and $T_{\alpha }\to T$ in WOT. For every trace-class S,

{\text{Tr}}\left(T_{\alpha }S\right)=\sum _{i}\lambda _{i}v_{i}^{*}\left(T_{\alpha }u_{i}\right)\longrightarrow \sum _{i}\lambda _{i}v_{i}^{*}\left(Tu_{i}\right)={\text{Tr}}(TS),

by invoking, for instance, the dominated convergence theorem.

Therefore every norm-bounded closed set is compact in WOT, by the Banach–Alaoglu theorem.

Other properties

The adjoint operation T → T*, as an immediate consequence of its definition, is continuous in WOT.

Multiplication is not jointly continuous in WOT: again let $T$ be the unilateral shift. Appealing to Cauchy-Schwarz, one has that both Tⁿ and T*ⁿ converges to 0 in WOT. But T*ⁿTⁿ is the identity operator for all $n$ . (Because WOT coincides with the σ-weak topology on bounded sets, multiplication is not jointly continuous in the σ-weak topology.)

However, a weaker claim can be made: multiplication is separately continuous in WOT. If a net T_i → T in WOT, then ST_i → ST and T_iS → TS in WOT.

SOT and WOT on B(X,Y) when X and Y are normed spaces

We can extend the definitions of SOT and WOT to the more general setting where X and Y are normed spaces and $B(X,Y)$ is the space of bounded linear operators of the form $T:X\to Y$ . In this case, each pair $x\in X$ and $y^{*}\in Y^{*}$ defines a seminorm $\|\cdot \|_{x,y^{*}}$ on $B(X,Y)$ via the rule $\|T\|_{x,y^{*}}=|y^{*}(Tx)|$ . The resulting family of seminorms generates the weak operator topology on $B(X,Y)$ . Equivalently, the WOT on $B(X,Y)$ is formed by taking for basic open neighborhoods those sets of the form

N(T,F,\Lambda ,\epsilon ):=\left\{S\in B(X,Y):\left|y^{*}((S-T)x)\right|<\epsilon ,x\in F,y^{*}\in \Lambda \right\},

where $T\in B(X,Y),F\subseteq X$ is a finite set, $\Lambda \subseteq Y^{*}$ is also a finite set, and $\epsilon >0$ . The space $B(X,Y)$ is a locally convex topological vector space when endowed with the WOT.

The strong operator topology on $B(X,Y)$ is generated by the family of seminorms $\|\cdot \|_{x},x\in X,$ via the rules $\|T\|_{x}=\|Tx\|$ . Thus, a topological base for the SOT is given by open neighborhoods of the form

N(T,F,\epsilon ):=\{S\in B(X,Y):\|(S-T)x\|<\epsilon ,x\in F\},

where as before $T\in B(X,Y),F\subseteq X$ is a finite set, and $\epsilon >0.$

Relationships between different topologies on B(X,Y)

The different terminology for the various topologies on $B(X,Y)$ can sometimes be confusing. For instance, "strong convergence" for vectors in a normed space sometimes refers to norm-convergence, which is very often distinct from (and stronger than) SOT-convergence when the normed space in question is $B(X,Y)$ . The weak topology on a normed space $X$ is the coarsest topology that makes the linear functionals in $X^{*}$ continuous; when we take $B(X,Y)$ in place of $X$ , the weak topology can be very different than the weak operator topology. And while the WOT is formally weaker than the SOT, the SOT is weaker than the operator norm topology.

In general, the following inclusions hold:

\{{\text{WOT-open sets in }}B(X,Y)\}\subseteq \{{\text{SOT-open sets in }}B(X,Y)\}\subseteq \{{\text{operator-norm-open sets in }}B(X,Y)\},

and these inclusions may or may not be strict depending on the choices of $X$ and $Y$ .

The WOT on $B(X,Y)$ is a formally weaker topology than the SOT, but they nevertheless share some important properties. For example,

(B(X,Y),{\text{SOT}})^{*}=(B(X,Y),{\text{WOT}})^{*}.

Consequently, if $S\subseteq B(X,Y)$ is convex then

{\overline {S}}^{\text{SOT}}={\overline {S}}^{\text{WOT}},

in other words, SOT-closure and WOT-closure coincide for convex sets.

References

v t e Banach space topics
Types of Banach spaces	Asplund Banach list Banach lattice Grothendieck Hilbert Inner product space Polarization identity (Polynomially) Reflexive Riesz L-semi-inner product (B Strictly Uniformly) convex Uniformly smooth (Injective Projective) Tensor product (of Hilbert spaces)
Banach spaces are:	Barrelled Complete F-space Fréchet tame Locally convex Seminorms/Minkowski functionals Mackey Metrizable Normed norm Quasinormed Stereotype
Function space Topologies	Banach–Mazur compactum Dual Dual space Dual norm Operator Ultraweak Weak polar operator Strong polar operator Ultrastrong Uniform convergence
Linear operators	Adjoint Bilinear form operator sesquilinear (Un)Bounded Closed Compact on Hilbert spaces (Dis)Continuous Densely defined Fredholm kernel operator Hilbert–Schmidt Functionals positive Pseudo-monotone Normal Nuclear Self-adjoint Strictly singular Trace class Transpose Unitary
Operator theory	Banach algebras C-algebras Operator space Spectrum C-algebra radius Spectral theory of ODEs Spectral theorem Polar decomposition Singular value decomposition
Theorems	Anderson–Kadec Banach–Alaoglu Banach–Mazur Banach–Saks Banach–Schauder (open mapping) Banach–Steinhaus (Uniform boundedness) Bessel's inequality Cauchy–Schwarz inequality Closed graph Closed range Eberlein–Šmulian Freudenthal spectral Gelfand–Mazur Gelfand–Naimark Goldstine Hahn–Banach hyperplane separation Kakutani fixed-point Krein–Milman Lomonosov's invariant subspace Mackey–Arens Mazur's lemma M. Riesz extension Parseval's identity Riesz's lemma Riesz representation Robinson-Ursescu Schauder fixed-point Sobczyk's theorem
Analysis	Abstract Wiener space Banach manifold bundle Bochner space Convex series Differentiation in Fréchet spaces Derivatives Fréchet Gateaux functional holomorphic quasi Integrals Bochner Dunford Gelfand–Pettis regulated Paley–Wiener weak Functional calculus Borel continuous holomorphic Measures Lebesgue Projection-valued Vector Weakly / Strongly measurable function
Types of sets	Absolutely convex Absorbing Affine Balanced/Circled Bounded Convex Convex cone (subset) Convex series related ((cs, lcs)-closed, (cs, bcs)-complete, (lower) ideally convex, (Hx), and (Hwx)) Linear cone (subset) Radial Radially convex/Star-shaped Symmetric Zonotope
Subsets / set operations	Affine hull (Relative) Algebraic interior (core) Bounding points Convex hull Extreme point Interior Linear span Minkowski addition Polar (Quasi) Relative interior
Examples	Absolute continuity AC $ba(\Sigma )$ c space Banach coordinate BK Besov $B_{p,q}^{s}(\mathbb {R} )$ Birnbaum–Orlicz Bounded variation BV Bs space Continuous C(K) with K compact Hausdorff Hardy H^p Hilbert H Morrey–Campanato $L^{\lambda ,p}(\Omega )$ ℓ^p $\ell ^{\infty }$ L^p $L^{\infty }$ weighted Schwartz $S\left(\mathbb {R} ^{n}\right)$ Segal–Bargmann F Sequence space Sobolev W^k,p Sobolev inequality Triebel–Lizorkin Wiener amalgam $W(X,L^{p})$
Applications	Differential operator Finite element method Mathematical formulation of quantum mechanics Ordinary Differential Equations (ODEs) Validated numerics

v t e Hilbert spaces
Basic concepts	Adjoint Inner product and L-semi-inner product Hilbert space and Prehilbert space Orthogonal complement Orthonormal basis
Main results	Bessel's inequality Cauchy–Schwarz inequality Riesz representation
Other results	Hilbert projection theorem Parseval's identity Polarization identity (Parallelogram law)
Maps	Compact operator on Hilbert space Densely defined Hermitian form Hilbert–Schmidt Normal Self-adjoint Sesquilinear form Trace class Unitary
Examples	Cⁿ(K) with K compact & n<∞ Segal–Bargmann F

v t e Duality and spaces of linear maps
Basic concepts	Dual space Dual system Dual topology Duality Operator topologies Polar set Polar topology Topologies on spaces of linear maps
Topologies	Norm topology Dual norm Ultraweak/Weak-* Weak polar operator in Hilbert spaces Mackey Strong dual polar topology operator Ultrastrong
Main results	Banach–Alaoglu Mackey–Arens
Maps	Transpose of a linear map
Subsets	Saturated family Total set
Other concepts	Biorthogonal system