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# apr25 - Math 4124 Monday April 25 April 25 Ungraded...

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Unformatted text preview: Math 4124 Monday, April 25 April 25, Ungraded Homework Exercise 7.5.2 on page 264 Let R be an integral domain and let D be a multiplicatively closed subset of R which contains 1 but not 0. Prove that the ring of fractions D- 1 R is isomorphic to a subring of the quotient field of R (hence is also an integral domain). Let S = R- 0 and let φ : R → S- 1 R denote the natural homomorphism defined by φ ( r ) = r / 1. Note that S- 1 R is the quotient field of the integral domain R , and φ is a monomorphism. In this situation we usually identify R with φ ( R ) , but we won’t do so here. Since φ ( d ) 6 = for all d ∈ D , we see that φ ( d ) is invertible for all d ∈ D . By the universal property for D- 1 R , we may extend φ to a map α : D- 1 R → S- 1 R . Specifically if θ : R → D- 1 R is the natural homomorphism defined by θ ( r ) = r / 1, then φ = αθ . All that remains to prove is that α is a monomorphism, equivalently ker α = 0, so suppose r / d ∈ ker α where r ∈...
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apr25 - Math 4124 Monday April 25 April 25 Ungraded...

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