What is the relationship between sup of unbounded sets in real numbers?

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The discussion centers on the properties of the supremum (sup) of unbounded sets in real numbers. It establishes that the supremum of the empty set is defined as negative infinity, while the supremum of a set V that is not bounded above is positive infinity. The proof presented demonstrates that if V is a subset of W, then the supremum of V is less than or equal to the supremum of W, using a proof by contrapositive to illustrate the contradiction that arises when assuming otherwise.

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  • Understanding of supremum and infimum in real analysis
  • Familiarity with proof techniques, particularly proof by contrapositive
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  • Basic concepts of intervals in the context of real numbers
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1. sup (empty set) = -infinity, and if V is not bounded above, then sup V = +infinity. Prove if V[tex]\subseteq[/tex]W[tex]\subseteq[/tex]Real Numbers then sup V is lessthan/equalto supW




3. I used a proof by contrapositive, but I'm not sure if it is completely valid...
 
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I am assuming that these are intevals. You know that the lemma is clear if V=W, don't you? So assume that
[tex] V\subset W<br /> [\tex]<br /> So there is an element w in W which is not an element of V, examine |w-sup(V)|.[/tex]
 
No reason to assume these are intervals.

Yes, the contrapositive is the way to go. Suppose sup(W)> sup(V). Then there exist x such that sup(V)< x< sup(W). From that it follows that there exist a member of W larger than x and so larger than any member of V, a contradiction.
 

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