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A linear operator : between two topological vector spaces (TVSs) is called a bounded linear operator or just bounded if whenever is bounded in then () is bounded in . A subset of a TVS is called bounded (or more precisely, von Neumann bounded ) if every neighborhood of the origin absorbs it.
Thus a unitary operator is a bounded linear operator that is both an isometry and a coisometry, [1] or, equivalently, a surjective isometry. [2] An equivalent definition is the following: Definition 2. A unitary operator is a bounded linear operator U : H → H on a Hilbert space H for which the following hold: U is surjective, and
A T ∈ L(H) is a Fredholm operator if and only if T is invertible modulo compact perturbation, i.e. TS = I + C 1 and ST = I + C 2 for some bounded operator S and compact operators C 1 and C 2. In other words, an operator T ∈ L(H) is Fredholm, in the classical sense, if and only if its projection in the Calkin algebra is invertible.
Since the graph of T is closed, the proof reduces to the case when : is a bounded operator between Banach spaces. Now, factors as / .Dually, ′ is ′ () ′ ′ (/ ) ′ ′.
In operator theory, a bounded operator T: X → Y between normed vector spaces X and Y is said to be a contraction if its operator norm ||T || ≤ 1. This notion is a special case of the concept of a contraction mapping, but every bounded operator becomes a contraction after suitable scaling. The analysis of contractions provides insight into ...
In functional analysis, the open mapping theorem, also known as the Banach–Schauder theorem or the Banach theorem [1] (named after Stefan Banach and Juliusz Schauder), is a fundamental result that states that if a bounded or continuous linear operator between Banach spaces is surjective then it is an open map.
The first inequality (that is, ‖ ‖ < for all ) states that the functionals in are pointwise bounded while the second states that they are uniformly bounded. The second supremum always equals ‖ ‖ (,) = ‖ ‖, ‖ ‖ and if is not the trivial vector space (or if the supremum is taken over [,] rather than [,]) then closed unit ball can be replaced with the unit sphere
Let H be a Hilbert space, L(H) be the bounded operators on H, and V ∈ L(H) be an isometry.The Wold decomposition states that every isometry V takes the form = for some index set A, where S is the unilateral shift on a Hilbert space H α, and U is a unitary operator (possible vacuous).