supremum equals to infumum The question is:
What can you say about the set M if sup M = inf M.
I know that supremum is the lowest upper bound and infumum is the biggest lower bound. But I cant figure out what you can say about the set M. And we only had one lession about supremum and infumum.

You know that for any $x \in M$, we have $\inf M \leq x \leq \sup M$, by definition of upper and lower bounds (not necessarily least upper or greatest lower bound). Now, if $\inf M = \sup M$, what does that say about the possible values of $x$?
