 The space of test functions on "U" is also denoted is also D("U") or formula_45.So to define the space of distributions, which is denoted by formula_46, we must first define the topology on the space formula_44, which requires that several other topological vector spaces (TVSs) be defined first.To define the various special subspaces of formula_46 (such as distributions of order formula_49 some integer, etc.