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Unformatted text preview: Algebra , Assignment 9 Solutions 1. Let R be a ring. Call a R a unit if ab = 1 for some b R . Let X be the set of units Prove that X forms a group under multiplication. What is our standard notation for X in the case where R = Z n ? Solution: Identity: As 1 1 = 1, 1 X . Product: Let a, c X . Then there exist b, d R with ab = cd = 1. Then ( ac )( bd ) = ( ab )( cd ) = 1 1 = 1 so ac X . Inverse: Let a X . Then there exists b R with ab = 1. But then b X as there exists an element (namely, a ) whose product with b is 1. 2. Recall Z [ 2] = { a + b 2 : a, b Z } . Find a unit (as defined above) a + b 2 Z [ 2] which has a 10. (One approach: Find some unit other than 1 and apply previous problem.) Solution: By a combination of luck, skill and diligence you notice that ( 2 + 1)( 2 1) = 1 so that = 1 + 2 is a unit. But then any power of is a unit. 2 = 3+2 2, 3 = 7+5 2 and 4 = 17+12 2 is as desired. (There are other solutions as well.) 3. Let Z [ i ], as usual, denote the Gaussian Integers and set I equal the multiples of 2 + i . (That is, I is all numbers (2 + i ) where Z [...
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 Fall '08
 JOSEPHS
 Algebra, Multiplication

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