# Proofs of the four type of intervals

1. Dec 1, 2016

### Kernul

1. The problem statement, all variables and given/known data
Verify that if an interval is bounded it must be one of the four following types: $(a, b)$, $[a, b]$, $(a, b]$, or $[a, b)$.

2. Relevant equations

3. The attempt at a solution
I don't quite get what I should actually prove here.
Do I have to see if, for $A \subseteq S$, we have $\forall x \in S, a < x < b$, with $a$ and $b$ being the lower and upper bound of $A$? (this is for the first type $(a, b)$)

2. Dec 1, 2016

### Staff: Mentor

How does your book define "bounded interval"?

For an interval of finite length, each endpoint either is or is not included in the interval.

3. Dec 1, 2016

### Kernul

The book says "A given subset of R, and in particular an interval, is called bounded if it has both an upper and lower bound."
So I have to see if the endpoints $a$ and $b$ are the lower and the upper bound of the interval?

4. Dec 1, 2016

### Staff: Mentor

You can assume that a is the lower bound and b is the upper bound. Now, is a included in the interval or is it not included? Is b included in the interval or is it not included? How many different interval types are we talking about here?

5. Dec 1, 2016

### Staff: Mentor

What is an interval?

6. Dec 1, 2016

### Kernul

Oh, so I start by assuming they are already a lower and upper bound and then prove if they are included in the interval or not. This means that I have to prove if the inequalities are $<$ or $\leq$.
The types are 4, right? Unless we just take the first and the third, since the other two are basically the opposite of the others.

An interval is a subset $I$ of $\mathbb{R}$ where whenever $a$ and $b$ belong to $I$ and $a \leq c \leq b$, then $c$ also belongs to $I$.

7. Dec 1, 2016

### Staff: Mentor

I would start with a given set $S$ that is bounded, i.e. there are numbers $A$ and $B$ such that $A \leq s \leq B$ for all $s\in S$.
Next I would define supremum and infimum of these bounds, say $B_S$ and $A_I$, and proceed by what @Mark44 has said: they are either in or out of $S$. Finally there should be an argument, why there cannot be holes between $B_S$ and $A_I$, i.e. elements outside of $S$.

8. Dec 1, 2016

### Staff: Mentor

I'm not sure that a full-fledged proof is required here.
Because of this wording, I believe "verify that ... " does not require the same rigor as "prove that ..." would need.
Posting in the Precalc section, the OP might not be familiar with these terms.

9. Dec 1, 2016

### Kernul

I get it. So I would find out if $B_S$ and $A_I$ are in or out of this set $S$.
The argument you're talking about is the axiom of completeness, right?

So, what should I do when it says "verify that"? Simply see if they are in the interval or not?
Supremum and infimum are, respectively, the least upper bound and the greatest lower bound, right?

10. Dec 1, 2016

### Staff: Mentor

I'm not sure, if we really needed completeness, because I think, it should work for rational numbers as well: the left and right points of an interval are either in our out. But I might have overlooked a loophole for there are some really nasty sets out there. $B_S$ is simply the lowest upper bound of all possible and $A_I$ the largest lower bound of all possible. Completeness makes it easy, yes.

11. Dec 1, 2016

### Staff: Mentor

IMO, not needed for this problem.
"they" being the endpoints -- how many ways can you specify an interval of finite length (i.e., a bounded interval) where the two endpoints either are or are not include? I believe that's all this problem is asking for.

12. Dec 1, 2016

### Kernul

By saying that the endpoints are equal or greater than the lower bound and equal or lower than the upper bound.
Let's say $a, b$ are the endpoints. The least upper bound is $L$ and the greatest lower bound is $l$. Then I should have $a \geq l$ and $b \leq L$. Am I right?

13. Dec 1, 2016

### Staff: Mentor

No, that's not at all what I am saying, which has nothing to do with least upper bounds or greatest lower bounds. If you have an interval, is the left endpoint included? Yes or no?
Is the right endpoint included? Yes or no?
The yes/no answers determine four kinds of intervals.

14. Dec 1, 2016

### Kernul

Oh. So it's really just that.
Sorry, I think I was trying to do something so simple in a complicated way.
Thank you.