i added the question in the link:
this is a total abstract thing
i dont know how to prove SUP whithout a formula to the series
i need to prove that S is the least upper bound
i dont know how to do that. i can say that
there is a cetain "e"
that if we subtract it from S we will get a member from A which is larger then S.
but is it a proof??
and what the -> thing means in the end ??