9.16. Let R be a ring and I a proper ideal. 1. If R is an integral domain, does it follow that R/I is an integral domain? Prove that it does, or find a counterexample. 2. If R/I is an integral domain, does it follow that R is an integral domain? Prove that it does, or find a counterexample.

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter6: More On Rings
Section6.1: Ideals And Quotient Rings
Problem 30E: a. For a fixed element a of a commutative ring R, prove that the set I={ar|rR} is an ideal of R....
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Could you explain how to show 9.16 in detail? I included list of theorems and definitions from the textbook.

Definition 9.3. Let R be a ring and I an ideal of R. Then the factor ring (or quotient
ring), R/I, is the set of all left cosets {a + I : a e R} together with the operations
(a + I) + (b +I) = a +b+ I and (a + I)(b+ I) = ab + I, for all a, b e R.
Theorem 9.6. For any ring R and ideal I, the factor ring R/I is a ring.
Example 9.9. Let R = Z and I = (5) = 5Z. Then R/I = {0+ I, 1 + I, 2 +
I,3 + 1,4 + I} and, for instance, (2 + I) + (4 + I)
(3 + I)(4+ I) = 12 + I = 2 +I.
= 6+ I = 1 + I and
%3D
Example 9.10. Let R = M2(Z) and let I be the ideal consisting of all matrices
whose entries are even. Then notice that for any a;; e Z, we have
aji a12
b11 b12
+1 =
+1,
a21 a22
b21 b22,
where bij is 0 if a¡; is even and 1 if a;j is odd. Thus, R/I consists of the sixteen
different elements (bii b12
b21 b22
+1,b¡¡ € {0, 1}. We perform arithmetic in the following
fashion:
((49) +*) * (6 ) + +) – (; 2) +- - (")-
(6:) + ') = (; )
+ I
1 0
+ 1) +
+1:
and
(::) -·) (6) +-) - (; ) + -(;) --
)(:) --) - (:)
+ I.
1 2
Example 9.11. Let R = R[x] and I = (x² + 3). Readers familiar with polynomial
long division will know that if f (x) e R, then f (x) = (x² + 3)q(x) +r(x), where
q and r are polynomials, with r(x)
unfamiliar with polynomial long division can peek ahead to Section 10.1, where it
will be discussed in more generality.) Since (x² + 3)q(x) e I by absorption, we
know that elements of R/I are of the form a + bx + I, with a, b e R. Addition
behaves as expected; for instance, (2+3x + I)+(7 – 4x + I) = 9 – x+ I. To deal
with multiplication, observe that x2 – (-3) e I; thus, x² + I = -3+I. Therefore,
we have calculations such as
= a + bx, for some a, b E R. (Those who are
(5 + 4x + I)(-7+2x + I) = -35 – 18x + 8x² + I
= -35 – 18x + 8(-3) + I
= -59 – 18x + I.
Let us also record a few basic facts about factor rings.
Theorem 9.7. Let R be a ring and I an ideal. Then
1. if R is commutative, then so is R/I;
2. if R has an identity, then so does R/I; and
3. if u is a unit of R, then u + I is a unit of R/I.
Theorem 9.8. Let R be a ring with ideals I and J, such that I C J. Then J/I is
an ideal of R/I.
Transcribed Image Text:Definition 9.3. Let R be a ring and I an ideal of R. Then the factor ring (or quotient ring), R/I, is the set of all left cosets {a + I : a e R} together with the operations (a + I) + (b +I) = a +b+ I and (a + I)(b+ I) = ab + I, for all a, b e R. Theorem 9.6. For any ring R and ideal I, the factor ring R/I is a ring. Example 9.9. Let R = Z and I = (5) = 5Z. Then R/I = {0+ I, 1 + I, 2 + I,3 + 1,4 + I} and, for instance, (2 + I) + (4 + I) (3 + I)(4+ I) = 12 + I = 2 +I. = 6+ I = 1 + I and %3D Example 9.10. Let R = M2(Z) and let I be the ideal consisting of all matrices whose entries are even. Then notice that for any a;; e Z, we have aji a12 b11 b12 +1 = +1, a21 a22 b21 b22, where bij is 0 if a¡; is even and 1 if a;j is odd. Thus, R/I consists of the sixteen different elements (bii b12 b21 b22 +1,b¡¡ € {0, 1}. We perform arithmetic in the following fashion: ((49) +*) * (6 ) + +) – (; 2) +- - (")- (6:) + ') = (; ) + I 1 0 + 1) + +1: and (::) -·) (6) +-) - (; ) + -(;) -- )(:) --) - (:) + I. 1 2 Example 9.11. Let R = R[x] and I = (x² + 3). Readers familiar with polynomial long division will know that if f (x) e R, then f (x) = (x² + 3)q(x) +r(x), where q and r are polynomials, with r(x) unfamiliar with polynomial long division can peek ahead to Section 10.1, where it will be discussed in more generality.) Since (x² + 3)q(x) e I by absorption, we know that elements of R/I are of the form a + bx + I, with a, b e R. Addition behaves as expected; for instance, (2+3x + I)+(7 – 4x + I) = 9 – x+ I. To deal with multiplication, observe that x2 – (-3) e I; thus, x² + I = -3+I. Therefore, we have calculations such as = a + bx, for some a, b E R. (Those who are (5 + 4x + I)(-7+2x + I) = -35 – 18x + 8x² + I = -35 – 18x + 8(-3) + I = -59 – 18x + I. Let us also record a few basic facts about factor rings. Theorem 9.7. Let R be a ring and I an ideal. Then 1. if R is commutative, then so is R/I; 2. if R has an identity, then so does R/I; and 3. if u is a unit of R, then u + I is a unit of R/I. Theorem 9.8. Let R be a ring with ideals I and J, such that I C J. Then J/I is an ideal of R/I.
9.16. Let R be a ring and I
a
proper
ideal.
1. If R is an integral domain, does it follow that R/I is an integral domain? Prove
that it does, or find a counterexample.
2. If R/I is an integral domain, does it follow that R is an integral domain? Prove
that it does, or find a counterexample.
Transcribed Image Text:9.16. Let R be a ring and I a proper ideal. 1. If R is an integral domain, does it follow that R/I is an integral domain? Prove that it does, or find a counterexample. 2. If R/I is an integral domain, does it follow that R is an integral domain? Prove that it does, or find a counterexample.
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