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compact operator (Definition)

Let $ X$ and $ Y$ be two Banach spaces. A compact operator (completely continuous operator) is a linear operator $ T \colon X \to Y$ that maps the unit ball in $ X$ to a set in $ Y$ with compact closure. It can be shown that a compact operator is necessarily a bounded operator.

The set of all compact operators on $ X$, commonly denoted by $ \mathbb{K}(X)$, is a closed two-sided ideal of the set of all bounded operators on $ X$, $ \mathbb{B}(X)$.

Any bounded operator which is the norm limit of a sequence of finite rank operators is compact. In the case of Hilbert spaces, the converse is also true. That is, any compact operator on a Hilbert space is a norm limit of finite rank operators.

Example 1 (Integral operators)
Let $ k(x,y)$, with $ x,y \in [0,1]$, be a continuous function. The operator defined by
$\displaystyle (T\psi)(x) = \int_0^1 k(x,y) \psi(y) \,\d y, \qquad \psi \in C([0,1])$
is compact.



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Other names:  completely continuous

Attachments:
approximation property (Definition) by asteroid
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Cross-references: continuous function, converse, Hilbert spaces, finite rank, sequence, limit, norm, two-sided ideal, closed, bounded operator, closure, compact, unit ball, maps, linear operator, operator, Banach spaces
There are 11 references to this entry.

This is version 5 of compact operator, born on 2004-06-26, modified 2008-06-29.
Object id is 5966, canonical name is CompactOperator.
Accessed 5850 times total.

Classification:
AMS MSC46B99 (Functional analysis :: Normed linear spaces and Banach spaces; Banach lattices :: Miscellaneous)

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