Let R and S be commutative rings with unity. (a) Let I be an
ideal of R and let J be an ideal of S. Prove that I × J = {(a, b) |
a ∈ I, b ∈ J} is an ideal of R × S. (b) (Harder!) Let L be any
ideal of R × S. Prove that there exists an ideal I of R and an
ideal J of S such that L = I × J.
Let R be a commutative domain, and let I be a prime ideal of
R.
(i) Show that S defined as R \ I (the complement of I in R) is
multiplicatively closed.
(ii) By (i), we can construct the ring R1 =
S-1R, as in the course. Let D = R / I. Show that
the ideal of R1 generated by I, that is,
IR1, is maximal, and R1 / I1R is
isomorphic to the
field of fractions of...
Let R be a commutative ring with identity with the property that
every ideal in R is principal. Prove that every homomorphic image
of R has the same property.
Let R be a ring (not necessarily commutative), and let X denote
the set of two-sided ideals
of R.
(i) Show that X is a poset with respect to to set-theoretic
inclusion, ⊂.
(ii) Show that with respect to the operations I ∩ J and I + J
(candidates for meet and join;
remember that I+J consists of the set of sums, {i + j} where i ∈
I and j ∈ J) respectively,
X is a lattice.
(iii) Give...
A field is a commutative ring with
unity in which every nonzero element is a unit.
Question: Show that Z_5 under
addition and multiplication mod 5 is a field. (state the
operations, identities, inverses)
For an arbitrary ring R, prove that a) If I is an ideal of R,
then I[ x] forms an ideal of the polynomial ring R[ x]. b) If R and
R' are isomorphic rings, then R[ x] is isomorphic to R' [ x ].
10.3.6 Exercise: Product of Pairwise Comaximal Ideals. Let R be
a commutative ring, and let {A1,...,An} be a pairwise comaximal set
ofn ideals. Prove that A1 ···An = A1 ∩ ··· ∩ An. (Hint: recall that
A1 ···An ⊆ A1 ∩···∩An from 8.3.8).