to restate part (ii). Lemma 1.3.8. Assume s ER is an upper bound for a set A CR. Then, sup A if and only if, for every choice of e> 0, there exists an element a Є A satisfying s - € < a. 18 Chapter 1. The Real Numbers Proof. Here is a short rephrasing of the lemma: Given that s is an upper bound, s is the least upper bound if and only if any number smaller than s is not an upper bound. Putting it this way almost qualifies as a proof, but we will expand on what exactly is being said in each direction. () For the forward direction, we assume s = sup A and consider s-e, where € > 0 has been arbitrarily chosen. Because s-e 0 is chosen, s-e is no longer an upper bound for A. Notice that what this implies is that if b is any number less than s, then b is not an upper bound. (Just let e = s-b.) To prove that s = sup A, we must verify part (ii) of Definition 1.3.2. (Read it again.) Because we have just argued that any number smaller than s cannot be an upper bound, it follows that if b is some other upper bound for A, then s sup B. (b) A finite set that contains its infimum but not its supremum. (c) A bounded subset of Q that contains its supremum but not its infimum. Exercise 1.3.3. (a) Let A be nonempty and bounded below, and define B = {bЄR: b is a lower bound for A}. Show that sup B = inf A. (b) Use (a) to explain why there is no need to assert that greatest lower bounds exist as part of the Axiom of Completeness. Exercise 1.3.4. Let A1, A2, A3,... be a collection of nonempty sets, each of which is bounded above.
to restate part (ii). Lemma 1.3.8. Assume s ER is an upper bound for a set A CR. Then, sup A if and only if, for every choice of e> 0, there exists an element a Є A satisfying s - € < a. 18 Chapter 1. The Real Numbers Proof. Here is a short rephrasing of the lemma: Given that s is an upper bound, s is the least upper bound if and only if any number smaller than s is not an upper bound. Putting it this way almost qualifies as a proof, but we will expand on what exactly is being said in each direction. () For the forward direction, we assume s = sup A and consider s-e, where € > 0 has been arbitrarily chosen. Because s-e 0 is chosen, s-e is no longer an upper bound for A. Notice that what this implies is that if b is any number less than s, then b is not an upper bound. (Just let e = s-b.) To prove that s = sup A, we must verify part (ii) of Definition 1.3.2. (Read it again.) Because we have just argued that any number smaller than s cannot be an upper bound, it follows that if b is some other upper bound for A, then s sup B. (b) A finite set that contains its infimum but not its supremum. (c) A bounded subset of Q that contains its supremum but not its infimum. Exercise 1.3.3. (a) Let A be nonempty and bounded below, and define B = {bЄR: b is a lower bound for A}. Show that sup B = inf A. (b) Use (a) to explain why there is no need to assert that greatest lower bounds exist as part of the Axiom of Completeness. Exercise 1.3.4. Let A1, A2, A3,... be a collection of nonempty sets, each of which is bounded above.
Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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