5. Let R be a commutative ring with identity and a e R. O a- O show that + ax is a unit in R[3), [HnE: Consider || – a (O) Fa'- On, show that, + axisa unit in R(3). 6. Let R be a commutative ring with identity and a e R. If l + ax is a unit in R(x), show that a - Og for some integer n>0. [Hint: Suppose that the inverse of la+ ax is by + bx + byx +...+ b. Since their product is l, bo- la (Why?) and the other coefficients are all O)

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#15 on the book. 

b #15 on the book | bartleby
Thomas W. Hungerford - Abstrac X
O File | C:/Users/angel/Downloads/Thomas%20W.%20Hungerford%20-%20Abstract%20Algebra_%20AN%20lntroduction-Cengage%20Learning%20(2014).pdf
A' Read aloud V Draw
F Highlight
O Erase
117
of 621
Capyr 2012 e La AI i a A May aot be A nd or hle or in pert D te edoie d, ed perty eom Ony bepmed mte Bodk endarChee. Hdtalnd t
ded tay ed co d t ty ate oad gmpat C Lag right o eddenal co g tit dt re i
4.2 Divisibility in F[x]
95
Ceurestu
(b) Give an example in Z[x] to show that part (a) may be false if the leading
coefficient of g(x) is not a unit. [Hint: Exercise 5(b) with Z in place of Q.]
15. Let R be a commutative ring with identity and a e R.
(a) If a² = 0g, show that 1g + ax is a unit in R[x]. [Hint: Consider I – ax +
(b) If a = OR, show that 1g + ax is a unit in R[x].
16. Let R be a commutative ring with identity and a E R. If 1g + ax is a unit in
R[x], show that d = Og for some integer n> 0. [Hint: Suppose that the inverse
of 1g+ ax is bo + bjx + bzx² +...+ bx. Since their product is 1g, bo = 1r
(Why?) and the other coefficients are all 0g-]
17. Let R be an integral domain. Assume that the Division Algorithm always
holds in R[x). Prove that R is a field.
18. Let p:R[x] R be the function that maps each polynomial in R[x] onto its
constant term (an element of R). Show that o is a surjective homomorphism
of rings.
19. Let ç:Z[x] →Z_[x] be the function that maps the polynomial a, + ajx + . ..+
ax in Z[x] onto the polynomial [ao] + [aj]x + • . · + [a]x*, where [a] denotes
the class of the integer a in Z,. Show that o is a surjective homomorphism of
rings.
20. Let D:R[x]–→ R[x] be the derivative map defined by
D(a, + ax + az + qx) = a, + 2a,x + 3a,x +...+ na,-!.
Is Da homomorphism of rings? An isomorphism?
C.21. Let h:R-→ S be a homomorphism of rings and define a function h:R[x] -> S[x]
by the rule
h(a, + ajx + ... + apx") = h(as) + h(aj)x + h(az)x² + ... + h(a,)x".
Prove that
(a) h is a homomorphism of rings.
(b) h is injective if and only if h is injective.
(c) A is surjective if and only if h is surjective.
(d) If R = S, then R[x] = S[x].
22. Let R be a commutative ring and let k(x) be a fixed polynomial in R[x]. Prove
that there exists a unique homomorphism ç:R[x] → R[x] such that
o(r) = r for all r eR
and
p(x) = k(x).
11:39 AM
e Type here to search
EPIC
Ai
EPIC
99+
10/30/2020
Transcribed Image Text:b #15 on the book | bartleby Thomas W. Hungerford - Abstrac X O File | C:/Users/angel/Downloads/Thomas%20W.%20Hungerford%20-%20Abstract%20Algebra_%20AN%20lntroduction-Cengage%20Learning%20(2014).pdf A' Read aloud V Draw F Highlight O Erase 117 of 621 Capyr 2012 e La AI i a A May aot be A nd or hle or in pert D te edoie d, ed perty eom Ony bepmed mte Bodk endarChee. Hdtalnd t ded tay ed co d t ty ate oad gmpat C Lag right o eddenal co g tit dt re i 4.2 Divisibility in F[x] 95 Ceurestu (b) Give an example in Z[x] to show that part (a) may be false if the leading coefficient of g(x) is not a unit. [Hint: Exercise 5(b) with Z in place of Q.] 15. Let R be a commutative ring with identity and a e R. (a) If a² = 0g, show that 1g + ax is a unit in R[x]. [Hint: Consider I – ax + (b) If a = OR, show that 1g + ax is a unit in R[x]. 16. Let R be a commutative ring with identity and a E R. If 1g + ax is a unit in R[x], show that d = Og for some integer n> 0. [Hint: Suppose that the inverse of 1g+ ax is bo + bjx + bzx² +...+ bx. Since their product is 1g, bo = 1r (Why?) and the other coefficients are all 0g-] 17. Let R be an integral domain. Assume that the Division Algorithm always holds in R[x). Prove that R is a field. 18. Let p:R[x] R be the function that maps each polynomial in R[x] onto its constant term (an element of R). Show that o is a surjective homomorphism of rings. 19. Let ç:Z[x] →Z_[x] be the function that maps the polynomial a, + ajx + . ..+ ax in Z[x] onto the polynomial [ao] + [aj]x + • . · + [a]x*, where [a] denotes the class of the integer a in Z,. Show that o is a surjective homomorphism of rings. 20. Let D:R[x]–→ R[x] be the derivative map defined by D(a, + ax + az + qx) = a, + 2a,x + 3a,x +...+ na,-!. Is Da homomorphism of rings? An isomorphism? C.21. Let h:R-→ S be a homomorphism of rings and define a function h:R[x] -> S[x] by the rule h(a, + ajx + ... + apx") = h(as) + h(aj)x + h(az)x² + ... + h(a,)x". Prove that (a) h is a homomorphism of rings. (b) h is injective if and only if h is injective. (c) A is surjective if and only if h is surjective. (d) If R = S, then R[x] = S[x]. 22. Let R be a commutative ring and let k(x) be a fixed polynomial in R[x]. Prove that there exists a unique homomorphism ç:R[x] → R[x] such that o(r) = r for all r eR and p(x) = k(x). 11:39 AM e Type here to search EPIC Ai EPIC 99+ 10/30/2020
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