(a) Prove that −A defines a cut.(b) What goes wrong if we set −A = {r ∈ Q : −r /∈ A}?(c) If a ∈ A and r ∈ −A, show a + r ∈ O. This shows A + (−A) ⊆ O. Now,finish the proof of property (f4) for addition in Definition 8.6.4.Although the ideas are similar, the technical difficulties increase when wetry to create a definition for multiplication in R. This is largely due to the factthat the product of two negative numbers is positive. The standard method ofattack is first to define multiplication on the non-negative cuts.Given A ≥ O and B ≥ O in R, define the productAB = {ab : a ∈ A, b ∈ B with a, b ≥ 0} ∪ {q ∈ Q : q < 0}.

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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(a) Prove that −A defines a cut.
(b) What goes wrong if we set −A = {r ∈ Q : −r /∈ A}?
(c) If a ∈ A and r ∈ −A, show a + r ∈ O. This shows A + (−A) ⊆ O. Now,
finish the proof of property (f4) for addition in Definition 8.6.4.
Although the ideas are similar, the technical difficulties increase when we
try to create a definition for multiplication in R. This is largely due to the fact
that the product of two negative numbers is positive. The standard method of
attack is first to define multiplication on the non-negative cuts.
Given A ≥ O and B ≥ O in R, define the product
AB = {ab : a ∈ A, b ∈ B with a, b ≥ 0} ∪ {q ∈ Q : q < 0}.

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