# Ring Theory Problems. A σ

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1 Ring Theory Problems 1. Given the commutative diagram α A σ B β A σ B show that α: ker σ ker σ and that β : coker σ coker σ. Here coker σ = B/σ(A). 2. Let K be a field, let V be an infinite dimensional K-vector space with basis {v 0, v 1, v 2,...}, and set R = End K (V ). Prove that R R = RR R R by constructing a 2 1 invertible R-matrix using suitable shift maps. 3. Given the commutative diagram α A B C D E β γ δ ɛ A B C D E with exact rows and with α, β, δ and ɛ isomorphisms. Prove that γ is an isomorphism. This is the five lemma. 4. Let R = Z. If F R is free, show that F contains no infinitely divisible element. Conclude that F does not contain Q R and hence that Q R is not projective. 5. Again let R = Z, set V = Z, a countable direct product of copies of Z, and let W = Z be the direct sum submodule. Suppose, by way of contradiction, that V is contained in a free Z-module F with basis B. (a) Show that there is a countable subset B of B so that W F F, where F has free basis B. (b) Notice that F/F is a free Z-module containing V/(V F ). (c) For each sequence ɛ = (ɛ 1, ɛ 2,...) of ± signs, define v ɛ V by v ɛ = i=1 ɛ ii! and show that v ɛ + (V F ) is infinitely divisible in F/F. (d) Conclude that V cannot be contained in a free Z-module. 6. Show that all right R-modules are free if and only if R is a division ring. 1

2 7. Given V R, show that there is a one-to-one correspondence between internal direct sums V = X Y and idempotents e in End(V R ). 8. A ring R is semihereditary if all finitely generated right ideals are projective R-modules. Show that, for such a ring R, any finitely generated submodule of a free module is projective. 9. Define R = ( ) Z Q, V = 0 Q ( ) 0 0, N = 0 Q ( ) 0 Q. 0 0 (a) Show that V and N are minimal right ideals of R, with V R projective and N R = VR. (b) Show that any right ideal I of R strictly larger than N + V is of the form zr for some nonzero integer z and hence I R = RR is projective. (c) If I is a right ideal of R and I does not contain N, observe that I + N = I N. In particular if I + N is projective, then so is I. (d) Conclude that R is right hereditary. (e) Let A = N + V, so that A is a 2-sided ideal of R. Prove that nr = A = AR and deduce that if F is a free left R-module, then nf = AF. (f) If RN is projective, then there is a free left R-module F with F = N W. Now note that N = nn nf = AF = AN AW = AW and this is a contradiction. Conclude that R is not left hereditary. 10. Let R be the ring of upper triangular n n matrices over a field K. (a) Show that each row of R is a projective right R-module. (b) Show that each row has a unique chain of R-submodules and that each submodule is projective. (c) Deduce that R is right hereditary. 11. Let a and b be positive integers that are not relatively prime and set R = Z/(ab). Show that the R-modules ar and br both have infinite projective dimension. 12. Let R be a ring that has no commutative homomorphic image and let A R and B R be right R-modules. Let φ: A B U be a map to the abelian group U that is bilinear and satisfies φ(ar, b) = φ(a, br) for all a A, b B and r R. Prove that φ is the 0 map. 2

3 13. Let I be an ideal of R and let A R and R B be R-modules with AI = 0 = IB. Show that A R B is naturally isomorphic to A R/I B. 14. Let I be a left ideal of R and let J be a right ideal. (a) If R I is flat, show that J I = JI. (b) If R/I is flat, prove that J I = JI. 15. Let R = K[x, y] be a polynomial ring in two variables over the field K and set I = Rx + Ry. Suppose by way of contradiction that R I is flat. (a) Deduce that I I = I 2 as an (R, R)-bimodule. (b) Note that I 2 /I 3 is the largest homomorphic image of I 2 that is annihilated on both sides by I and that dim K I 2 /I 3 = 3. (c) On the other hand, observe that I I maps onto (I/I 2 ) R (I/I 2 ) = (I/I 2 ) R/I (I/I 2 ) and this image has K-dimension Let R be a right Noetherian ring. (a) If the following is an exact sequence of finitely generated right R-modules 0 A 0 A 1 A 2 A n 0 prove that n i=0 ( 1)i [A i ] = 0 in G 0 (R). (b) If 0 = A 0 A 1 A 2 A n = A is a finite sequence of right submodules of the finitely generated module A, show that [A] = n i=1 [A i/a i 1 ] in G 0 (R). 17. The graded ring S = i=0 S i is said to be graded right Noetherian if the set of graded right ideals satisfies the ascending chain condition. Prove that S is right Noetherian if and only if it is graded right Noetherian. Note that if I is any right ideal of S and if I n denotes the set of degree n components of elements of I (S 0 + S S n ), then i=0 I i is a graded ideal of S. One can now proceed as in the proof of the Hilbert basis theorem. 18. Show that S 0 Noetherian does not imply that S is Noetherian. 19. Find all finitely generated graded K[x]-modules up to abstract isomorphism. Here K is a field and recall that K[x] is a PID. 20. In the following four problems we let J denote the Jacobson radical of the ring R. Prove Nakayama s lemma: Let V be a finitely generated R- module and let W be a submodule. (a) If W < V, show that there exists a 3

4 maximal submodule M of V with W M < V. (b) If V = W + V J, prove that V = W. In particular, if V = V J, conclude that V = Let P be a finitely generated projective R-module and let µ: V P be an R-module homomorphism. Let µ: V/V J P/P J be the natural map. (a) If µ is onto, conclude that P = µ(v ) + P J and then that µ is onto. (b) If µ is an isomorphism, use the splitting of µ: V P to deduce that µ is an isomorphism. 22. Suppose P is a finitely generated projective R-module, with P/P J a free R/J-module. (a) Show that there exists a finitely generated free R- module F with F/F J = P/P J. (b) Use the fact that P P/P J is an epimorphism and that F is projective, to construct a map µ: F P with µ: F/F J P/P J an isomorphism. (c) Conclude that P is free. 23. If all finitely generated projective R/J-modules are free, show that all finitely generated projective R-modules are free. 24. Let K be a field and set R = K[x x m = 0]. Show that the Cartan map c is not surjective. 25. Write A(a, b) and B(a, b) for the 4 4 triangular matrices a 0 0 a A(a, b) = b 0 0 b B(a, b) = b b a a with the entries arbitrary. Now let R and S be the four and eight dimensional subalgebras of M 4 (K) given by R = {A(a, b) a, b K} S = {B(a, b) a, b K} Observe that R/Rad(R) = K K = S/Rad(S) and describe the Cartan matrices of R and S. Conclude that c need not be injective in general. 26. Show that [X] = [Y ] in G 0 (R) if and only if there exist exact sequences 0 A B X C 0 and 0 A B Y C 0 with A, B, and C finitely generated R-modules. 4

5 27. If R is a right Noetherian ring, prove that the same is true of the power series ring R[[x]]. 28. Let R be a right Noetherian ring. Use Baer s Criterion to prove that a direct sum of injective R-modules is injective. 29. Let K be a field and suppose R is a K-algebra. Then any R-module V is a K-vector space on either side and thus V R is a (K, R)-bimodule. Show that the proof of Baer s Theorem carries over in this context using Hom K (, K) rather than Hom Z (, Q/Z). Conclude that if dim K R < and dim K V <, then V embeds in an injective R-module that is also finite dimensional over K. In particular, the injective hull of V is finite dimensional over K. ( ) K K 30. Let R =. Use the above to find E(e K 11 R), E(e 22 R) and E(R). 31. Let W i ess V i for i = 1, 2,.... Prove that i W i is essential in i V i. On the other hand, show by example that i W i need not be essential in i V i. 32. A ring R is self-injective if R R is an injective R-module. Use Baer s criterion to show that a Wedderburn ring is self-injective. Also show that Z/nZ is self-injective for any integer n If R is a ring with no zero divisors, prove that R is self-injective if and only if it is a division ring. 34. Let K be a field and set R = ( ) K K[x]. 0 K[x] Show that R has a classical right ring of quotients, but not a classical left ring of quotients. 5

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