operator topologies


Let X be a normed vector spacePlanetmathPlanetmath and B(X) the space of bounded operatorsMathworldPlanetmathPlanetmath in X. There are several interesting topologiesMathworldPlanetmathPlanetmath that can be given to B(X). In what follows, Tα denotes a net in B(X) and T denotes an element of B(X).

Note: On 4, 5, 6 and 7, X must be a Hilbert spaceMathworldPlanetmath.

0.1 1. Norm Topology

This is the topology induced by the usual operator norm.

TαTin the norm topologyTα-T0

0.2 2. Strong Operator Topology

This is the topology generated by the family of semi-norms x,xX defined by Tx:=Tx. That means

TαTin the strong operator topology(Tα-T)x0,xX

0.3 3. Weak Operator Topology

This is the topology generated by the family of semi-norms f,x, where xX and f is a linear functional of X (written fX*, the dual vector space of X), defined by Tf,x:=|f(Tx)|. That means

TαTin the weak operator topologyf((Tα-T)x)0,xX,fX*

In case X is an Hilbert space with inner product ,, we have that

TαTin the weak operator topology|(Tα-T)x,y|0,x,yX

0.4 4. σ-Strong Operator Topology

In this topology X must be a Hilbert space. Let K(X) denote the space of compact operatorsMathworldPlanetmath on X.

The σ-strong operator topology is the topology generated by the family of semi-norms S,SK(X), defined by TS:=TS. That means

TαTin the σ -strong operator topology(Tα-T)S0,SK(X)

Equivalently, TαTTαSTS in norm for every SK(X).

This topology is also called the ultra-strong operator topology.

0.5 5. σ-Weak Operator Topology

In this topology X must be a Hilbert space. Let B(X)* denote the space of trace-class operators on X and Tr(S) the trace of an operator SB(X)*.

The σ-weak operator topology is the topology generated by the family of semi-norms {ωS:SB(X)*} defined by ωS(T):=|Tr(TS)|. That means

TαTin the σ -weak operator topology|Tr[(Tα-T)S]|0,SB(X)*

This topology is also called the ultra-weak operator topology.

0.6 6. Strong-* Operator Topology

In this topology X must be a Hilbert space. In the following T* denotes the adjoint operator of T.

The strong-* operator topology is the topology generated by the family of semi-norms x,xX defined by Tx:=Tx+T*x. That means

TαTin the strong-* operator topology(Tα-T)x+(Tα*-T*)x0,xX

Equivalently, TαT if and only if TαxTx and Tα*xT*x, for every xX.

0.7 7. σ-Strong-* Operator Topology

In this topology X must be a Hilbert space. Let K(X) denote the space of compact operators on X. In the following T* denotes the adjoint operator of T.

The σ-strong-* operator topology is the topology generated by the family of semi-norms S,SK(X) defined by TS:=TS+T*S. That means

TαTin the σ -strong-* operator topology(Tα-T)S+(Tα*-T*)S0,SK(X)

Equivalently, TαT if and only if TαSTS and Tα*ST*S in norm, for every SK(X).

This topology is also called ultra-strong-* operator topology.

0.8 Comparison of Operator Topologies

  • The norm topology is the strongest of the topologies defined above.

  • The weak operator topology is weaker than the strong operator topology, which is weaker than the norm topology.

  • In Hilbert spaces we can summarize the relations of the above topologies in the following diagram. Given two topologies 𝒰,𝒱 the notation 𝒰𝒱 means 𝒰 is weaker than 𝒱:

    \xymatrixweak\ar[r]\ar[d]&strong\ar[r]\ar[d]&strong-*\ar[d] σ -weak\ar[r]& σ -strong\ar[r]& σ -strong-*\ar[r]&Norm
Title operator topologies
Canonical name OperatorTopologies
Date of creation 2013-03-22 17:22:04
Last modified on 2013-03-22 17:22:04
Owner asteroid (17536)
Last modified by asteroid (17536)
Numerical id 18
Author asteroid (17536)
Entry type Definition
Classification msc 54E99
Classification msc 47L05
Classification msc 46A32
Related topic OperatorNorm
Defines strong operator topology
Defines weak operator topology
Defines σ-weak operator topology
Defines σ-strong operator topology
Defines strong-* operator topology
Defines σ-strong-* operator topology
Defines ultra-strong operator topology
Defines ultra-weak operator topology
Defines ultra-stro