Approximation Property
In mathematics, a Banach space is said to have the approximation property (AP), if every compact operator is a limit of finite-rank operators. The converse is always true.
Every Hilbert space has this property. There are, however, Banach spaces which do not; Per Enflo published the first counterexample in a 1973 article. However, a lot of work in this area was done by Grothendieck (1955).
Later many other counterexamples were found. The space of bounded operators on does not have the approximation property (Szankowski). The spaces for and (see Sequence space) have closed subspaces that do not have the approximation property.
Read more about Approximation Property: Definition, Examples
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—Mark Twain [Samuel Langhorne Clemens] (1835–1910)