# prove or disprove: sup of two sets

• January 21st 2013, 04:48 AM
Stormey
prove or disprove: sup of two sets
Hi,
i need to prove or disprove the following:

$Sup(A\cup B)=max\left \{ SupA, SupB \right \}$

*edit:
both sets are bounded from above.
• January 21st 2013, 05:10 AM
emakarov
Re: prove or disprove: sup of two sets
Prove (1) $\sup(A\cup B)\le\max(\sup A,\sup B)$ and (2) $\sup(A\cup B)\ge\max(\sup A,\sup B)$.

For (1), show that $\max(\sup A,\sup B)$ is an upper bound of $A\cup B$.

For (2), use the fact that $x\ge y$ and $x\ge z$ imply $x\ge\max(y,z)$.
• January 21st 2013, 07:53 AM
johng
Re: prove or disprove: sup of two sets
I've found the following to be useful:
supremums of larger sets are larger and infimums of larger sets are smaller

Formally, (assuming the sets are bounded):
1. If $C\subseteq D$ then $sup(C)\leq sup(D)$
2. If $C\subseteq D$ then $inf(C)\geq inf(D)$

Proof of 1 - Let $u=sup(D)$. Let $x\in C$, then $x\in D$ and so $x\leq u$; i.e. $u$ is an upper bound of $C$. So $sup(C)\leq u$. The proof for infs is exactly similar.

For your original question, since $A\subseteq A\cup B$, $sup(A)\leq sup(A\cup B)$, and similarly for B.
• January 21st 2013, 09:06 AM
Stormey
Re: prove or disprove: sup of two sets
Thanks guys.