# Thread: Supremum not an element of set, prove infinite

1. ## Supremum not an element of set, prove infinite

Let A be a nonempty subset of the Reals bounded above. Suppose sup A exists and that sup A is not an element of A. Show that A contains a countably infinite subset. In particular, A is infinite.

I'm reasonably sure that the easiest way to prove this is using an epsilon argument, but I don't really know where to start on here.

Here's my scrambled thoughts:

Let a = sup A
x be an element of A
x <= a

For any epsilon > 0 there is a-epsilon < x < a
how do a I show that the interval from a-epsilon to x is countably infinite?

Thanks

2. ## Re: Supremum not an element of set, prove infinite

Originally Posted by jsmith90210
Let A be a nonempty subset of the Reals bounded above. Suppose sup A exists and that sup A is not an element of A. Show that A contains a countably infinite subset. In particular, A is infinite.

I'm reasonably sure that the easiest way to prove this is using an epsilon argument, but I don't really know where to start on here.

Here's my scrambled thoughts:

Let a = sup A
x be an element of A
x <= a

For any epsilon > 0 there is a-epsilon < x < a
how do a I show that the interval from a-epsilon to x is countably infinite?

Thanks
I think the best way to do this is to do the easy proof that if $A=\{x_1,\cdots,x_n\}$ with $x_1\leqslant\cdots\leqslant x_n$ then $\sup A=x_n$ and so $\sup A\in A$. So, for $\sup A\notin A$ one MUST have that $A$ is not finite. Sound feasible? Or do you want to do the more direct proof?

3. ## Re: Supremum not an element of set, prove infinite

I see the logic in your proof, but if possible, I'd like a more direct proof

4. ## Re: Supremum not an element of set, prove infinite

Originally Posted by jsmith90210
Let A be a nonempty subset of the Reals bounded above. Suppose sup A exists and that sup A is not an element of A. Show that A contains a countably infinite subset. In particular, A is infinite.
Suppose $\alpha=\sup(A)~\&~\alpha\notin A.$.
$\left( {\exists x_1 \in A} \right)\left[ {\alpha - 1 < x_1 < \alpha } \right]$ WHY?

If $n\ge 2$ then $\left( {\exists x_n \in A} \right)\left[ {n_{n-1} < x_n < \alpha } \right]$. WHY?

How does that prove it?

5. ## Re: Supremum not an element of set, prove infinite

Should that be xn-1 < xn? And I don't know how that proves it, that's the part I'm struggling with

6. ## Re: Supremum not an element of set, prove infinite

Originally Posted by jsmith90210
Should that be xn-1 < xn? And I don't know how that proves it, that's the part I'm struggling with
1) if $x<\alpha$ then it is not an upper bound. So?

2) we have $x_1
That is an infinite collection.