HOMEWORK 4
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HOMEWORK 4
Definition. If I and J are ideals in a ring R, then we define
I + J = { a + b | a ∈ I,b ∈ J }.
Exercise 1. Prove that if I and J are ideals in a ring R, then I + J is an ideal which contains both I and J.
Definition. If R is a commutative ring with identity and a ∈ R, then
(a) = { ra | r ∈ R }
denotes the principal ideal generated by a.
Exercise 2. Let 0 a,b ∈ Z and let d be the greatest common divisor of a and b. Prove that
(a) + (b) = (d).
Exercise 3. Use the First Isomorphism Theorem to prove that
Z20 /(5) Z5 .
Exercise 4. Let R be an integral domain and let a, b ∈ R. Prove that if ( a) = (b), then there exists a unit u ∈ R such that a = bu.
2023-02-28