Def 16.4 ∅ is open Def pg. 327 A subset 𝐺 of ℝ is open set in ℝ if for each 𝑥∈𝐺 there exits a neighborhood 𝑉 of 𝑥 that 𝑉⊆𝐺. A subset 𝐹 of ℝ is closed in ℝ if the complement 𝐶(𝐹):=ℝ\𝐹 is open in ℝ