# supremum

• Jan 13th 2011, 11:32 AM
luckylawrance
supremum
let inf S=m and sup S=M.
if T={|x-y| :x,y belong to S},
then prove that sup T= M-m
• Jan 13th 2011, 12:16 PM
Drexel28
Quote:

Originally Posted by luckylawrance
let inf S=m and sup S=M.
if T={|x-y| :x,y belong to S},
then prove that sup T= M-m

What ideas do you have?
• Jan 13th 2011, 12:20 PM
CSM
Intuitively the difference between to elements in S is the biggest when one is the biggest element while the other one is the smallest (or vice versa).
• Jan 14th 2011, 04:30 AM
luckylawrance
Respected, the idea i have is that difference between two numbers is greatest when one is largest and other is smallest in S. but i don't know how to write the proof in proper way.
• Jan 15th 2011, 03:54 PM
Drexel28
Quote:

Originally Posted by luckylawrance
Respected, the idea i have is that difference between two numbers is greatest when one is largest and other is smallest in S. but i don't know how to write the proof in proper way.

Use the characterization that if $A\subseteq\mathbb{R}$ is bounded then $\sup A=\alpha$ if and only if $\alpha$ is an upper bound of $A$ and for every $\varepsilon>0$ there exists $a\in A$ such that $\alpha-\varepsilon<\alpha$.