Topology, Proofs, The word "Complement" 1. The problem statement, all variables and given/known data I have a proof to do in which they use the word "complement". I am not sure what it means by that withing the context of the question. There is no glossary to the book and there is no mention of complement before this question. 2. Relevant equations Show that the complement of the closed interval [a,b] is an open set. 3. The attempt at a solution I have not tried it yet because I do not know what they mean by complement. If you could point me to the right website where it talks about this or give a good definition I will then try and finish the proof from there.