Def 17.1 ̄𝑆={lim𝑘𝑥𝑘∣𝑥𝑘∈𝑆}∪𝑆 is closure of set 𝑆, it has following properties ̄𝑆 is closed ̄𝑆 is smallest closed set which contain set 𝑆 Def 18.1 𝑆 is dense in ̄𝑆 (∀𝑦∈̄𝑆)(∃𝑥∈𝑆)(∀𝜀>0)𝑑(𝑥,𝑦)<𝜀