View Single Post

## Proving a property of real numbers

1. The problem statement, all variables and given/known data

Given x<y for some real numbers x and y. Prove that there is at least
one real z satisfying x<z<y

2. Relevant equations

This is an exercise from Apostol's Calculus Vol. 1. The usual laws of
algebra, inequalities, a brief discussion on supremum, infimum and
the Archimedean property preceeded this exercise.

3. The attempt at a solution

I'm trying to solve as many problems from the first chapter, which I had
just glossed over during a first read. Proving theorems in a rigorous
fashion is not exactly my forte. It becomes all the more difficult when
I'm asked to prove something "intuitively obvious."

I think I have the solution in fragments.

Let S be the set of all numbers greater than x. x is a lower bound for
this set, and therefore S has a greatest lower bound - inf S.

$$inf S \geq x$$

[The following argument assumes that x = inf S]

Consider some $$y \in S$$. I can always find at least one $$z \in S$$ such that z < y, because if there are no z's satisfying that inequality then y would be inf S.

The only problem here seems to be to prove that inf S = x. The first choice is a proof by contradiction.

Assume inf S > x...

Am I on the right track?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
 PhysOrg.com science news on PhysOrg.com >> Galaxies fed by funnels of fuel>> The better to see you with: Scientists build record-setting metamaterial flat lens>> Google eyes emerging markets networks