ABSTRACT
To specify a topology on a set is to describe certain subsets that will play the role of neighborhoods. These sets are called open sets.
In what follows, we will use “interval notation”: If a ≤ b are real numbers then we define
(a, b) = {x ∈ R : a < x < b} , [a, b] = {x ∈ R : a ≤ x ≤ b} , [a, b) = {x ∈ R : a ≤ x < b} , (a, b] = {x ∈ R : a < x ≤ b} .
