Marginal implies unconditionally closed
This article gives the statement and possibly, proof, of an implication relation between two subgroup properties. That is, it states that every subgroup satisfying the first subgroup property (i.e., marginal subgroup) must also satisfy the second subgroup property (i.e., unconditionally closed subgroup)
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Suppose is a T0 topological group (i.e., a topological group whose underlying set is a T0 space) and is a marginal subgroup of as an abstract group. Then, is a closed subgroup of (i.e., it is a closed subset in the topological sense). In fact, is a closed normal subgroup of .
In particular, the result applies to the cases that is a Lie group.
- Marginal implies algebraic (the intermediate property used here is algebraic subgroup).
- Algebraic implies unconditionally closed
The proof follows by combining Facts (1) and (2).