Let S,T be non-empty bounded subsets of the real numbers. Prove that is bounded above.

How would you start this question?

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- Jan 2nd 2011, 10:54 AMworc3247Suprema question
Let S,T be non-empty bounded subsets of the real numbers. Prove that is bounded above.

How would you start this question? - Jan 2nd 2011, 11:37 AMFernandoRevilla

If are upper bounds for and respectively, prove that for all .

Fernando Revilla - Jan 2nd 2011, 11:45 AMKevin123
That`s not right, you should take max{s,t} as upper bound. For example think of S=T=[-2,-1] and s=t=-1. In this case s+t=-2 is no upper bound of the union.

- Jan 2nd 2011, 12:01 PMFernandoRevilla
It is right, I said

upper bounds, not minimum upper bounds.

Quote:

you should take max{s,t}

:)

Fernando Revilla - Jan 2nd 2011, 12:07 PMKevin123
Oh sorry, I did not see that you assume s,t >= 0. Of course then your answer is right... :-)

- Jan 2nd 2011, 12:14 PMworc3247
I think I can see the general idea, but am having trouble writing a formal proof. Here is what I have thus far:

Let and such that are upper bounds for S and T respectively.

If we set max{ }, then:

. Therefore is bounded above.

Is this along the right lines or am I way off? - Jan 2nd 2011, 12:33 PMKevin123
You cannot assume that the upper bound is in S. Think of S=(0,1). Then any upper bound is greater or equal 1.

- Jan 2nd 2011, 12:39 PMworc3247
Ok, so I need to set and so that they are greater than the supremum of the sets?