# Almost open linear map

In functional analysis and related areas of mathematics, an almost open linear map between topological vector spacess (TVSs) is a linear operator that satisfies a condition similar to, but weaker than, the condition of being an open map.

## Definition

Let T : XY be a linear operator between two TVSs. We say that T is almost open if for any neighborhood U of 0 in X, the closure of T(U) in Y is a neighborhood of the origin.

Note that some authors call T is almost open if for any neighborhood U of 0 in X, the closure of T(U) in T(X) (rather than in Y) is a neighborhood of the origin; this article will not consider this definition.

If T : XY is a bijective linear operator, then T is almost open if and only if T−1 is almost continuous.

## Properties

Note that if a linear operator T : XY is almost open then because T(X) is a vector subspace of Y that contains a neighborhood of 0 in Y, T : XY is necessarily surjective. For this reason many authors require surjectivity as part of the definition of "almost open".

## Open mapping theorems

Theorem: If X is a complete pseudometrizable TVS, Y is a Hausdorff TVS, and T : XY is a closed and almost open linear surjection, then T is an open map.
Theorem: If T : XY is a surjective linear operator from a locally convex space X onto a barrelled space Y then T is almost open.
Theorem: If T : XY is a surjective linear operator from a TVS X onto a Baire space Y then T is almost open.
Theorem: Suppose T : XY is a continuous linear operator from a complete pseudometrizable TVS X into a Hausdorff TVS Y. If the image of T is non-meager in Y then T : XY is a surjective open map and Y is a complete metrizable space.