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Unformatted text preview: Math 302 Modern Algebra Spring 2009 Homework Solutions 3.1 DEFINITIONS AND EXAMPLES OF RINGS 2. Let R = { , e, b, c } with addition and multiplication defined by the following tables. It can be shown that R is a ring with this addition and multiplication. + e b c e b c e e c b b b c e c c b e e b c e e b c b b b c c c (a) Verify axioms R4 and R5. R4. Existence of Additive Identity. We need to show there exists an element 0 R R such that for all a R , a + 0 R = a. Proof: Choose 0 R = 0. Then 0 R R . We need to show ( a R ) [ a + 0 R = a ] . To show this, we can use a proof by exhaustion. From the table, 0 + 0 = 0 , 0 + e = e = e + 0 , 0 + b = b = b + 0 , 0 + c = c = c + 0 . Therefore a + 0 = a for all a R so 0 is an additive identity for R . Thus R satisfies R4. R5. Existence of Additive Inverses. We need to show for each a R , there exists x R such that a + x = 0 R . Proof: Use a proof by exhaustion. From the addition table, we have 0 + 0 = 0 , e + e = 0 , b + b = 0 , c + c = 0 , which implies that each element is its own additive inverse. Therefore R satisfies R5. (b) Is R commutative? R9. Commutative Law of Multiplication. We need to show for all x, y R , x y = y x. Proof: To check this, we again use a proof by exhaustion. We need to check all possible pairs of (distinct) elements from R : e = 0 = e b = 0 = b c = c = c e b = b = b e e c = b = c e b c = 0 = c b Therefore R satisfies R9 and so is a commutative ring. 23 3.1 Defnitions and Examples oF Rings Math 302 Homework Solutions page 24 2. (continued) (c) Is R a field? Since R is a commutative ring, to determine whether or not R is a field, we need to check axioms R10 and R12. R10. Existence of Multiplicative Identity. There exists an element 1 R R such that for all a R , a 1 R = a = 1 R a. Proof: This property is satisfied. Choose 1 R = e . Then 1 R R . We need to show ( a R ) [ a 1 R = a 1 R a = a ] . To show this, we can use a proof by exhaustion. From the table, e = 0 = e , e e = e, b e = b = e b, c e = c = e c. Therefore e is a multiplicative identity for R , so R satisfies R10 and R is a ring with identity. R12. Existence of Multiplicative Inverses. For each a R , a 6 = 0 R = 0, there exists x R such that a x = 1 R = e . Proof: This property is not satisfied. For both b and c , there does not exist an element in R such that b x = e and c x = e . So neither of these elements has a multiplicative inverse. Therefore R is not a field....
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This note was uploaded on 10/20/2009 for the course MATH 302 taught by Professor Edwards during the Spring '03 term at CSU Fullerton.
 Spring '03
 Edwards
 Algebra, Addition, Multiplication

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