# Finding sup, inf, max and min for each sets

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## Solution 1

1) $\inf (-3, -1) =3$ because $\forall x \le -3| x \not \in (-3,-1)$ so $-3$ is lower bound. If $x > -3$ then there is an $e$ so that $-3<e <x$ and $e \in (-3,-1)$ so $-3$ is greatest lower bound.

There is not $\min$ of $(-3,-1)$ so for and $x \in (-3,-1)$ we can find an $e$ so that $-3 < e < x < -1$ so $e \in (-3-1)$.

Same reasoning: we can determine $\sup (-3,-1) = -1$ and there is no max.

Or we could say "there is not square bracket". I guess. That is how the square bracket was meant to be defined.

2) $\inf\{1/n| n\in \mathbb N\} = 0$ because: $0 < 1/n \forall n \in \mathbb N$ so $0$ is a lower bound. If $\epsilon > 0$ we can find an $n \in \mathbb N$ so that $0 < 1/n < \epsilon$. (Why? Archimedean principal. But you can take it as a given usually, I think.) So $0$ is the greatest lower bound.

There is no minimum as for all $1/n \in$ the set, $1/(n+1)$ is in the set and smaller.

As $1 > 1/2 > 1/3 > ...... > 1/n > 1/(n+ 1).....$, $\max$ of set is $1$. $\sup = 1$ because, if maximum exists then all members of the set are smaller than the maximum and anything smaller than the maximum will have the maximum larger than it. So $\sup$ must equal $\max$ if $\max$ exists.

3) $\{1/n| n \in \mathbb Z; n \ne 0\}$

For ever $1/n > 0$ in the set $-1/n$ is also in that set (and, of course, -1/n < 0 < 1/n). So $\max = \sup = 1$ by the same argument above and $\min = \inf = -1$.

4) $S= \{ x < 6|x \in \mathbb Q\}$

$\sup = 6$ because $6$ is an upper bound (all $x \in S$ are such $x < 6$ by definition) and for any $x < 6$ there is a rational number $q$ such that $x < q < 6$. So $6$ is least upper bound.

There is no maximum as $6 \not \in S$.

$S$ is not bounded below as for all real $n$ we can find a rational $q$ such that $q < n$. So there is no minimum nor is there any $\inf$.

5) $S= \{ 1/n + (-1)^n\}$

It might be worthwhile listing a few of these. For even $n$ we have 1 1/2, 1 1/4, 1 1/6, etc. and for odd we have 0, -2/3, -4/5 etc. Okay, got it.

Okay $3/2$ is both the max and the sup. because 1)for $n = 1$, $1/n + (-1)^n = 0$. For $n \ge 2$ we have $1/n + (-1)^n \le 1/n + 1 \le 3/2$. So $3/2$ is an upper bound and as $3/2 \in S$ it is the least upper bound and the maximum value.

$-1 = \inf S$ because. $-1 < -1 + 1/n \le 1/n + (-1)^n$ so $-1$ is lower bound. For any $x > -1$ we can find $0 < 1/(n+1) + 1/n < \epsilon = x - (-1)$. Thus $-1 < -1 + 1/(n+ 1) < -1 + 1/n < x$. One of $n$ or $n+1$ must be odd and so one of $n$ or $n+1$ must be is S. So $x$ is not a lower bound. So $-1$ is the greatest lower bound.

There is no minimum as $-1 \not \in S$. (There is no $1/n = 0$ so there is no $1/n + (-1)^n = -1$).

## Solution 2

• Set $2$: Supremum and maximum are $1$, infimum is $0$ and minimum does not exist.
• Set $3$: Maxmimum and supremum are $1$, infimum and minimum are $-1$.
• Set $5$: Infimum is $-1$, minimum does not exist, supremum and maximum are $\frac{3}{2}$.
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### Allie

studying actuarial

Updated on September 15, 2020

Set $$1$$ : $$(-3,-1)$$

Set $$2$$ : $$\{{\frac{1}{n} : n \in \mathbb{N}}\}$$

Set $$3$$ : $$\{{\frac{1}{n} : n \in \mathbb{Z}} , n \not= 0\}$$

Set $$4$$ : $$\{{r < 6 : r \in \mathbb{Q}}\}$$

Set $$5$$ : $$\{{\frac{1}{n}+(-1)^n: n \in \mathbb{N}}\}$$

TRIED: I have these $$5$$ sets as listed. I have figured that for Set $$1$$, sup = $$-1$$, inf = $$-3$$, and max/min doesn't exist because it is not a squared bracket. As far as set $$4$$ is concerned, I figured that sup is $$6$$ where inf is negative infinity with max/min doesn't exist.

I am struggling with sets $$2,3,5$$ I feel like sup and inf for set $$2$$ is $$1,0$$ respectively but not quite sure if that is also its max and min. Also if I could get some help with sets $$3,5$$ that'd be wonderful. thank you.

Do you not need to supply rigorous proofs?
Set 2-5 are using "()" rather than "{}" for set notation. Should we be concerned? If we are to be consistent than set 1 should be {-3,-1} rather than $\{x| -3 < x < -1\}$.
One thing to note. You can only have one of the following three cases: Neither sup nor max exist; sup exists and max doesn't; max = sup. Same for inf and min.
Frustrating.. Isn't that? You need to "escape" by putting a back slash before them. \$\{ \}\$ will be $\{\}$. Escaping with backslash is a standard coding convention. The only one I can't figure out is ~. They disappear if you use them but if you escape them they get marked as invalid code????
Because 3/2 is in set 5 and all n>2, $\pm$1 + 1/n $\le$ 1 + 1/2.
For $n$ even, we have $\frac{1}{n} + 1$ and the largest value for $\frac{1}{n}$, where $n$ is even is $\frac{1}{2}$.