Math Refresher: Boundary Points
mathrefresher.blogspot.com › boundary-pointsSep 14, 2006 · Lemma 2: Every real number is a boundary point of the set of rational numbers Q. Proof: (1) A boundary point b by definition is a point where for any positive number ε, { b - ε , b + ε } contains both an element in Q and an element in Q'. (2) So all we need to show that { b - ε, b + ε } contains both a rational number and an irrational number.