We show that if
H is a dense subgroup of a
T3 semitopological group
G such that
H is a paratopological group, then
G is a paratopological group as well. Also, we prove that a semitopological group
G is a topological group provided
G is
T3 and it contains a dense topological subgroup.
We show that the functors T3 and Reg(G) do not respect dense subgroups of semitopological groups. This answers a problem posed by Mikhail Tkachenko.