In mathematics, an LF-space is a topological vector space V that is a locally convex inductive limit of a countable inductive system of Fréchet spaces. This means that V is a direct limit of the system in the category of locally convex topological vector spaces and each is a Fréchet space.
Original definition was also assuming that V is a strict locally convex inductive limit, which means that the topology induced on by is identical to the original topology on .
The topology on V can be described by specifying that an absolutely convex subset U is a neighborhood of 0 if and only if is an absolutely convex neighborhood of 0 in for every n.
An LF space is barrelled and bornological (and thus ultrabornological).
A typical example of an LF-space is, , the space of all infinitely differentiable functions on with compact support. The LF-space structure is obtained by considering a sequence of compact sets with and for all i, is a subset of the interior of . Such a sequence could be the balls of radius i centered at the origin. The space of infinitely differentiable functions on with compact support contained in has a natural Fréchet space structure and inherits its LF-space structure as described above. The LF-space topology does not depend on the particular sequence of compact sets .
With this LF-space structure, is known as the space of test functions, of fundamental importance in the theory of distributions.