K-space (functional analysis)

In mathematics, more specifically in functional analysis, a K-space is an F-space

such that every extension of F-spaces (or twisted sum) of the form

is equivalent to the trivial one[1]

is the real line.

are K-spaces,[1] as are all finite dimensional Banach spaces.

N. J. Kalton and N. P. Roberts proved that the Banach space