7.E: Intermediate and Extreme Values (Exercises)
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Q1
Mimic the definitions of an upper bound of a set and the least upper bound (supremum) of a set to give definitions for a lower bound of a set and the greatest lower bound (infimum) of a set.
Note: The infimum of a set S is denoted by inf(S).
Q2
Find the least upper bound (supremum) and greatest lower bound (infimum) of the following sets of real numbers, if they exist. (If one does not exist then say so.)
- S={1n|n=1,2,3,...}
- T={r|r is rational and r2<2}
- (−∞,0)∪(1,∞)
- R={(−1)nn|n=1,2,3,...}
- (2,3π] ∩ \mathbb{Q}
- The empty set \varnothing
Q3
Let S ⊆ R and let T = \{-x|x ∈ S\}.
- Prove that b is an upper bound of S if and only if -b is a lower bound of T.
- Prove that b = \sup S if and only if -b = \inf T.