Operator topologies

Template:Short description In the mathematical field of functional analysis there are several standard topologies which are given to the algebra Template:Math of bounded linear operators on a Banach space Template:Mvar.

IntroductionEdit

Let <math>(T_n)_{n \in \mathbb N}</math> be a sequence of linear operators on the Banach space <math>X</math>. Consider the statement that <math>(T_n)_{n \in \N}</math> converges to some operator <math>T</math> on <math>X</math>. This could have several different meanings:

If <math>\|T_n - T\| \to 0</math>, that is, the operator norm of <math>T_n - T</math> (the supremum of <math>\| T_n x - T x \|_X</math>, where <math>x</math> ranges over the unit ball in <math>X</math>) converges to <math>0</math>, we say that <math>T_n \to T</math> in the uniform operator topology.
If <math>T_n x \to Tx</math> for all <math>x \in X</math>, then we say <math>T_n \to T</math> in the strong operator topology.
Finally, suppose that for all <math>x \in X</math> we have <math>T_n x \to Tx</math> in the weak topology of <math>X</math>. This means that <math>F(T_n x) \to F(T x)</math> for all continuous linear functionals <math>F</math> on <math>X</math>. In this case we say that <math>T_n \to T</math> in the weak operator topology.

List of topologies on B(H)Edit

File:Optop.svg

Diagram of relations among topologies on the space Template:Math of bounded operators

There are many topologies that can be defined on Template:Math besides the ones used above; most are at first only defined when Template:Math is a Hilbert space, even though in many cases there are appropriate generalisations. The topologies listed below are all locally convex, which implies that they are defined by a family of seminorms.

In analysis, a topology is called strong if it has many open sets and weak if it has few open sets, so that the corresponding modes of convergence are, respectively, strong and weak. (In topology proper, these terms can suggest the opposite meaning, so strong and weak are replaced with, respectively, fine and coarse.) The diagram on the right is a summary of the relations, with the arrows pointing from strong to weak.

If Template:Mvar is a Hilbert space, the linear space of Hilbert space operators Template:Math has a (unique) predual <math>B(H)_*</math>, consisting of the trace class operators, whose dual is Template:Math. The seminorm Template:Math for w positive in the predual is defined to be Template:Math.

If Template:Mvar is a vector space of linear maps on the vector space Template:Mvar, then Template:Math is defined to be the weakest topology on Template:Mvar such that all elements of Template:Mvar are continuous.

The norm topology or uniform topology or uniform operator topology is defined by the usual norm ||x|| on Template:Math. It is stronger than all the other topologies below.
The weak (Banach space) topology is Template:Math, in other words the weakest topology such that all elements of the dual Template:Math are continuous. It is the weak topology on the Banach space Template:Math. It is stronger than the ultraweak and weak operator topologies. (Warning: the weak Banach space topology and the weak operator topology and the ultraweak topology are all sometimes called the weak topology, but they are different.)
The Mackey topology or Arens-Mackey topology is the strongest locally convex topology on Template:Math such that the dual is Template:Math, and is also the uniform convergence topology on Template:Math, Template:Math-compact convex subsets of Template:Math. It is stronger than all topologies below.
The σ-strong-^* topology or ultrastrong-^* topology is the weakest topology stronger than the ultrastrong topology such that the adjoint map is continuous. It is defined by the family of seminorms Template:Math and Template:Math for positive elements Template:Mvar of Template:Math. It is stronger than all topologies below.
The σ-strong topology or ultrastrong topology or strongest topology or strongest operator topology is defined by the family of seminorms Template:Math for positive elements Template:Mvar of Template:Math. It is stronger than all the topologies below other than the strong^* topology. Warning: in spite of the name "strongest topology", it is weaker than the norm topology.)
The σ-weak topology or ultraweak topology or weak-^* operator topology or weak-* topology or weak topology or Template:Math) topology is defined by the family of seminorms |(w, x)| for elements w of Template:Math. It is stronger than the weak operator topology. (Warning: the weak Banach space topology and the weak operator topology and the ultraweak topology are all sometimes called the weak topology, but they are different.)
The strong-^* operator topology or strong-^* topology is defined by the seminorms ||x(h)|| and ||x^*(h)|| for Template:Math. It is stronger than the strong and weak operator topologies.
The strong operator topology (SOT) or strong topology is defined by the seminorms ||x(h)|| for Template:Math. It is stronger than the weak operator topology.
The weak operator topology (WOT) or weak topology is defined by the seminorms |(x(h₁), h₂)| for Template:Math. (Warning: the weak Banach space topology, the weak operator topology, and the ultraweak topology are all sometimes called the weak topology, but they are different.)

Relations between the topologiesEdit

The continuous linear functionals on Template:Math for the weak, strong, and strong^* (operator) topologies are the same, and are the finite linear combinations of the linear functionals (xh₁, h₂) for Template:Math. The continuous linear functionals on Template:Math for the ultraweak, ultrastrong, ultrastrong^* and Arens-Mackey topologies are the same, and are the elements of the predual Template:Math.

By definition, the continuous linear functionals in the norm topology are the same as those in the weak Banach space topology. This dual is a rather large space with many pathological elements.

On norm bounded sets of Template:Math, the weak (operator) and ultraweak topologies coincide. This can be seen via, for instance, the Banach–Alaoglu theorem. For essentially the same reason, the ultrastrong topology is the same as the strong topology on any (norm) bounded subset of Template:Math. Same is true for the Arens-Mackey topology, the ultrastrong^*, and the strong^* topology.

In locally convex spaces, closure of convex sets can be characterized by the continuous linear functionals. Therefore, for a convex subset Template:Mvar of Template:Math, the conditions that Template:Mvar be closed in the ultrastrong^*, ultrastrong, and ultraweak topologies are all equivalent and are also equivalent to the conditions that for all Template:Math, Template:Mvar has closed intersection with the closed ball of radius Template:Mvar in the strong^*, strong, or weak (operator) topologies.

The norm topology is metrizable and the others are not; in fact they fail to be first-countable. However, when Template:Mvar is separable, all the topologies above are metrizable when restricted to the unit ball (or to any norm-bounded subset).

Topology to useEdit

The most commonly used topologies are the norm, strong, and weak operator topologies. The weak operator topology is useful for compactness arguments, because the unit ball is compact by the Banach–Alaoglu theorem. The norm topology is fundamental because it makes Template:Math into a Banach space, but it is too strong for many purposes; for example, Template:Math is not separable in this topology. The strong operator topology could be the most commonly used.

The ultraweak and ultrastrong topologies are better-behaved than the weak and strong operator topologies, but their definitions are more complicated, so they are usually not used unless their better properties are really needed. For example, the dual space of Template:Math in the weak or strong operator topology is too small to have much analytic content.

The adjoint map is not continuous in the strong operator and ultrastrong topologies, while the strong* and ultrastrong* topologies are modifications so that the adjoint becomes continuous. They are not used very often.

The Arens–Mackey topology and the weak Banach space topology are relatively rarely used.

To summarize, the three essential topologies on Template:Math are the norm, ultrastrong, and ultraweak topologies. The weak and strong operator topologies are widely used as convenient approximations to the ultraweak and ultrastrong topologies. The other topologies are relatively obscure.

ReferencesEdit

Functional analysis, by Reed and Simon, Template:ISBN
Theory of Operator Algebras I, by M. Takesaki (especially chapter II.2) Template:ISBN

Template:Banach spaces Template:Hilbert space Template:Duality and spaces of linear maps Template:Functional analysis Template:Topological vector spaces