Exercise 3: Let R be a commutative ring. (a) Let I and J be ideals in R. Show that In J is also an ideal in R. (b) Now let R be a principal ideal domain. Let a, b E R and let (a) (b) = (m) for some me R. Show that m is the least common multiple of a and b, written m = lcm(a, b), meaning that a | m, bm, and if c is an element such that ac and b | c, then m | c. [Note that m is defined only up to multiplication by a unit, by Exercise 1.] (c) Still assuming that R is a PID, suppose that (a, b) = (1) = R. Show that (a) n (b) = (ab). (Hint: You can write 1 = au + bv for some u, v € R.)

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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Exercise 3: Let R be a commutative ring.
(a) Let I and J be ideals in R. Show that In J is also an ideal in R.
(b) Now let R be a principal ideal domain. Let a, b = R and let (a)n(b) = (m)
for some m € R. Show that m is the least common multiple of a and b,
written m = lcm(a, b), meaning that a | m, b | m, and if c is an element
such that ac and b | c, then m | c. [Note that m is defined only up to
multiplication by a unit, by Exercise 1.]
(c) Still assuming that R is a PID, suppose that (a, b)
(1) = R. Show that
(a) (b) = (ab). (Hint: You can write 1 = au + bv for some u, v € R.)
=
Transcribed Image Text:Exercise 3: Let R be a commutative ring. (a) Let I and J be ideals in R. Show that In J is also an ideal in R. (b) Now let R be a principal ideal domain. Let a, b = R and let (a)n(b) = (m) for some m € R. Show that m is the least common multiple of a and b, written m = lcm(a, b), meaning that a | m, b | m, and if c is an element such that ac and b | c, then m | c. [Note that m is defined only up to multiplication by a unit, by Exercise 1.] (c) Still assuming that R is a PID, suppose that (a, b) (1) = R. Show that (a) (b) = (ab). (Hint: You can write 1 = au + bv for some u, v € R.) =
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