1. The problem statement, all variables and given/known data Show for any metric space (S,d) and e> 0 that N(e, S) <= D(e, S) <= N(e/2, S) 2. Relevant equations none 3. The attempt at a solution Can somebody please tell me what does N(e,S) or D(e,S) even mean so I can at least know what I am asked to prove? I dig through all the notes, and online sources and could not find anything operation that involves a metric space with a real number. Thanks!