>>83
>>R中のQのように稠密分散で、
>>R＼Qは、”a nonempty open set”の集まりになるけれども
>？

リウヴィル数をイメージしてもらえば、良いのでは？ 稠密分散で、”a nonempty open set”の集まり
例えば
Structure of the set of Liouville numbers より
”Each Un is an open set; as its closure contains all rationals (the p/q's from each punctured interval), it is also a dense subset of real line. Since it is the intersection of countably many such open dense sets, L is comeagre, that is to say, it is a dense Gδ set.”