Say we have a = \sup \{ a_{1}, a_{2}, a_{3}, ... \}. Then does this mean we can find some a_{n} \in \{ a_{1}, a_{2}, ... \} such that
|a - a_{n}| < \varepsilon
? My reasoning is that since a (the supremum) is the least upper bound of the set, we have to be able to find some member of the set...