# Math Help - Greatest lower and leat upper bound help

1. ## Greatest lower and leat upper bound help

what is the least upper bound and greatest lower bound of the set

$[-1,3] \cap (\sqrt{2} , 5]$ ?

${ { x \in {R} : e^{x} \leq 2 } }$

2. ## Re: Greatest lower and leat upper bound help

Originally Posted by Tweety
what is the least upper bound and greatest lower bound of the set

$[-1,3] \cap (\sqrt{2} , 5]$ ?

${ { x \in {R} : e^{x} \leq 2 } }$
I will start be admitting that I am new to this type of notation so if my answer sounds like nonsense then you are probably correct.

The intersection of those two intervals will be $(\sqrt2,3]$ I guess this is a limit on the x values

\begin{align*}e^x &\leq2\\lne^x & \leq ln2\\x&\leq ln2\\x&\leq 0.6931 approx\end{align*}

now if $x\leq ln2$ in the domain $(\sqrt{2} , 5]$ then there are no real solutions.

3. ## Re: Greatest lower and leat upper bound help

Originally Posted by Melody2
I will start be admitting that I am new to this type of notation so if my answer sounds like nonsense then you are probably correct.

The intersection of those two intervals will be $(\sqrt2,3]$ I guess this is a limit on the x values

\begin{align*}e^x &\leq2\\lne^x & \leq ln2\\x&\leq ln2\\x&\leq 0.6931 approx\end{align*}

now if $x\leq ln2$ in the domain $(\sqrt{2} , 5]$ then there are no real solutions.
I'm seeing 2 sets.

$\left.[-1,3]\cap \left(\sqrt{2},5\right.\right]$ and

{x : x is Real, ex <= 2}

OP doesn't specify any relation between these two sets so I think this is 2 problems.

$(\left.[-1,3]\cap \left(\sqrt{2},5\right.\right]$ = $\left.\left(\sqrt{2},3\right.\right]$ so

the LUB of the first set is clearly 3. The GLB is clearly $\sqrt{2}$

The LUB of the 2nd set = ln(2) as you noted. There is no lower bound on set 2 so there is no GLB.

4. ## Re: Greatest lower and leat upper bound help

Yes, I agree that this is two separate problems.

MelodyII is right that the first set is equivalent to the interval $(\sqrt{2}, 3]$ so its least upper bound is 3 and greatest lower bound is $\sqrt{2}$.

romset is, of course, using the fact that the logarithm is an increasing function: if b $a\le b$ then $ln(a)\le ln(b)$

So if $e^x\le 2$ then $x\le ln(2)$. So there is no greatest upper bound on x and the least upper bound is ln(2).

5. ## Re: Greatest lower and leat upper bound help

why is it called the least upper bound and not just the upper bound?
and
why is it called the greatest lower bound and not just the lower bound?

6. ## Re: Greatest lower and leat upper bound help

Originally Posted by Melody2
why is it called the least upper bound and not just the upper bound?
and why is it called the greatest lower bound and not just the lower bound?
Definition: $L$ is the least upper bound of a set $A$ means that $L$ is an upper bound of $A$ such that if $K$ is also an upper bound of $A$ then $L\le K$.

Let $A=[0.8)$ then $10$ is an upper bound of $A$ but $8=\text{LUB}(A).$