Suppose A and B are each bounded above. Define the set A+Bas follows A+B = {a +b: a E A, bE B} We claim that sup(A + B) < sup A+ sup B To prove this, we first let m = supA and n = supB. The existence of the supremums is guaranteed by axiom 5.2.4. We need to show that m + nis an upper bound for the set A + B. If this is true, then the least upper bound of A + B (which we know is sup(A+ B)) must be no larger than m + n. Now for any a E A, we know a < m, and for any b e B, we know b < n,hence a + b

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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**Suppose A and B are each bounded above. Define the set A + B as follows:**

\[ A + B = \{a + b: a \in A, \, b \in B\} \]

We claim that
\[ \sup(A + B) \leq \sup A + \sup B \]

To prove this, we first let \( m = \sup A \) and \( n = \sup B \). The existence of the supremums is guaranteed by axiom 5.2.4. We need to show that \( m + n \) is an upper bound for the set \( A + B \). If this is true, then the least upper bound of \( A + B \) (which we know is \( \sup(A + B) \)) must be no larger than \( m + n \). Now for any \( a \in A \), we know \( a \leq m \), and for any \( b \in B \), we know \( b \leq n \), hence \( a + b \leq m + n \) for any \( a + b \in A + B \). This shows that \( m + n \) is an upper bound for \( A + B \) and hence \( \sup(A + B) \leq m + n = \sup A + \sup B \).

*Using the notation above, show \(\inf(A + B) \geq \inf A + \inf B\).*

*Provide examples of sets A and B where strict inequality \(\sup(A + B) < \sup A + \sup B\) holds.*
Transcribed Image Text:**Suppose A and B are each bounded above. Define the set A + B as follows:** \[ A + B = \{a + b: a \in A, \, b \in B\} \] We claim that \[ \sup(A + B) \leq \sup A + \sup B \] To prove this, we first let \( m = \sup A \) and \( n = \sup B \). The existence of the supremums is guaranteed by axiom 5.2.4. We need to show that \( m + n \) is an upper bound for the set \( A + B \). If this is true, then the least upper bound of \( A + B \) (which we know is \( \sup(A + B) \)) must be no larger than \( m + n \). Now for any \( a \in A \), we know \( a \leq m \), and for any \( b \in B \), we know \( b \leq n \), hence \( a + b \leq m + n \) for any \( a + b \in A + B \). This shows that \( m + n \) is an upper bound for \( A + B \) and hence \( \sup(A + B) \leq m + n = \sup A + \sup B \). *Using the notation above, show \(\inf(A + B) \geq \inf A + \inf B\).* *Provide examples of sets A and B where strict inequality \(\sup(A + B) < \sup A + \sup B\) holds.*
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