Groups acting on themselves by left multiplication-Cayley’s theorem[Problems in the section 4.2 of ”Dummit and Foote: Abstract Algebra”]

These are  problems,  Its solutions will coming soon! Pdf file 42_problems


Solutions 42_solutions

Group actions and permutation representations[Problems in the section 4.1 of ”Dummit and Foote: Abtract Algebra”]

Let G be a group and let A be a nonempty set.

1. Let G act on the set A. Prove that if a,b\in A and b=g\cdot a for some g\in G, then G_b=gG_ag^{-1}($G_a$ is the stabilizer of a). Deduce that if G acts transitively on A then the kernel of the action is \cap_{g\in G}gG_ag^{-1}.

2. Let G be a permutation group on the set A(i.e., G\leq S_A), let \delta \in G and let a\in A. Prove that \delta G_a\delta^{-1}=G_{\delta(a)}. Deduce that if G acts transitively on A then \cap_{\delta \in G}\delta G_a\delta^{-1}=1.

3. Assume that G is an abelian, transitive supgroup of S_A. Show that \delta (a)\not =a\forall \delta\in G-\{1\}\forall a\in A. Deduce that |G|=|A|[Use the preceding exercise.]

4. Let S_3 act on the set \Omega of ordered pairs: \{(i,j)|1\leq i,j\leq 3\} by \delta ((i,j))=(\delta (i),\delta (j)). Find the orbits of S_3 on \Omega . For each \delta \in S_3 find the cycle decomposition of \delta under this action (i.e., find its cycle decomposition when \delta is considered as an element of S_9– first fix a labelling of these nine ordered pairs). For each orbit \mathcal{O} of S_3 acting on these nine points pick some a\in\mathcal{O} and find the stabilizer of a in S_3.

5. For each parts (a) and (b) repeat the preceding exercise but with S_3 action on the specified set:

(a)The set of 27 triples \{(i,j,k)|1\leq i,j,k\leq 3\}

(b)The set \mathcal{P}(\{1,2,3\})-\{\emptyset\} of all 7 nonempty subsets of \{1,2,3\}.

6. Let R be the set of all polynomials with integer coefficients in the independent variables x_1,x_2,x_3,x_4 and S_4 act on R by permuting the indices of the four variables:\sigma\cdot p(x_1,x_2,x_3,x_4)=p(x_{\sigma (1)},x_{\sigma (2)},x_{\sigma (3)},x_{\sigma (4)}) for all \sigma \in S_4 and p\in R.

a)Find the polynomials in the orbit of S_4 on R containing x_1+x_2;

b)Find the polynomials in the orbit of S_4 on R containing x_1x_2+x_3x_4;

c)Find the polynomials in the orbit of S_4 on R containing (x_1+x_2)(x_3+x_4).

7. Let G be a transitive permutation group on the finite set A. A block is a nonempty subset B of A such that for all \sigma \in G either \sigma (B)=B or \sigma (B)\cap B=\emptyset.

a)Prove that if B is a block containing the element a of A then G_B:=\{\sigma \in G|\sigma (B)=B\} is a subgroup of G containing G_a;

b)Show that if B is a block and \sigma_1(B),\cdots,\sigma_n(B) are all dinstinct images of B under the elements of G then these form a partion of A;

c)A transitive group G on a set A is said to be primitive if the only blocks in A are the trivial ones: the sets of size 1 and A itself. Show that S_4 is primitive on A=\{1,2,3,4\}. Show that D_8 is not primitive as a permutation group on the four vertices of a square;

d)Prove that the transitive group is primitive of A iff for each a\in A, the only subgroups of G containing G_a are G_a and G.

8. A transitive permutation group G on a set A is called doubly transitive if for any (hence all) a\in A the subgroup G_a is transitive on the set A-\{a\}.

a)Prove that S_n is doubly transitive on \{1,2,\cdots,n\} for all n>1;

b)Prove that a doubly transitive group is primitive. Deduce that D_8 is not doubly transitive in its action on the 4 vertices of a square.

9. Assume G acts transitively on the finite set A and let H be a normal subgroup of G. Let \mathcal{O}_1,\mathcal{O}_2,\cdots,\mathcal{O}_r be the distinct orbits of H on A.

a)Prove that G permutes the sets \mathcal{O}_i. Prove that G is transitive on \{\mathcal{O}_i\}. Deduce that all orbits of H on A have the same cardinality;

b)Prove that if a\in\mathcal{O}_1 then |\mathcal{O}_1|=|H:H\cap G_a| and r=|G:HG_a|.

10. Let H and K be subgroups of the group G. For each x\in G define the HK double coset of x in G to be the set HxK=\{hxk|h\in H, k\in K\}.

a)Prove that HxK is the union of the left cosets x_iK, where \{x_iK\} is the orbit containing xK of H acting by left multiplication on the set of left cosets of K;

b)Prove that HxK is the union of right cosets of H;

c)Prove that HxK and HyK are either the same set or are disjoint for all x,y\in G. Show that the set of HK double cosets partitions G;

d)Prove that |HxK|=|K|\cdot |H:H\cap xKx^{-1}|;

e)Prove that |HxK|=|H|\cdot |K:K\cap x^{-1}Hx|.

P.S. These problems are from ”Dummit and Foote, Abstract Algebra”. Solutions will coming soon! 😀

Pdf file: 41_problems



Kiểm tra Đại số hiện đại, ngày 12-11-2008 (thày Phùng Hồ Hải)

Bài 1. Cho V là không gian các đa thức có bậc nhỏ hơn n với hệ số phức, ở đây n>1 là một số nguyên. Xét toán tử đạo hàm d:V\to V. Tồn tại hay không một toán tử g:V\to V là đa thức theo d và thoả mãn V là tổng trực tiếp của Im(d) và Im(g).

Bài 2. V là không gian véc tơ hữu hạn chiều bất kỳ trên \mathbb{C}f:V\to V là một ánh xạ tuyến tính.

a)Chứng minh rằng tồn tại các ánh xạ tuyến tính f_s,f_n:V\to V thoả mãn f_s chéo hoá được, f_n luỹ linh, f_s,f_n là các đa thức theo ff=f_s+f_n.

b) Chứng minh rằng cặp ánh xạ trên là duy nhất.

Bài 3. Cho k là một trường có đặc số 0. Xét đại số nhóm k[S_n] với cơ sở e_{\sigma},\sigma\in S_n. Chứng minh rằng x^2=x với x=\dfrac{1}{n!}\sum_{\sigma\in S_n}\epsilon (\sigma)e_{\sigma}. (\epsilon (\sigma) là dấu của hoán vị \sigma)

Bài 4. Cho f:M\to M là một đồng cấu modun thoả mãn f^2=f. Chứng minh rằng tồn tại modun con N của M để M là tổng trực tiếp của Im(f) và N.

Bài 5. Bằng định nghĩa tích tensor chứng minh rằng \mathbb{Z}/(m,n)\mathbb{Z} đẳng cấu với tích tensor của hai nhóm abel \mathbb{Z}/m\mathbb{Z}\mathbb{Z}/n\mathbb{Z}.

Bài tập Đại số hiện đại, 30-10-2008(thày Phùng Hồ Hải)

Bài 1. Tính Hom_{\mathbb{Z}}(\mathbb{Z},\mathbb{Q}); Hom_{\mathbb{Z}}(\mathbb{Q},\mathbb{Z}); Hom_{\mathbb{Z}}(\mathbb{Z}_n,\mathbb{Z}_m);Hom_{\mathbb{Z}}(\mathbb{Z}_n,\mathbb{Q})Hom_{\mathbb{Z}}(\mathbb{Q},\mathbb{Z}_n).

Bài 2. Cho A là nhóm Abel hữu hạn sinh. Chứng minh rằng Hom_{\mathbb{Z}}(A,\mathbb{Q}/\mathbb{Z})\not =0. Khi nào thì có đơn cấu từ A đến \mathbb{Q}/\mathbb{Z}?

Bài 3. Cho vành R và M là một R-modun đơn. Chứng minh rằng End_R(M) là một thể.

Bài 4. V là K-không gian véc tơ hữu hạn chiều, R=End_K(V). Coi V là modun trên R. Tính End_R(V).

Bài giảng về đại số Lie và nhóm Lie

Introduction to Lie Algebras and Lie Groups

Fall 2008


International Master Class

Institute of Mathematics

Vietnam Academy of Science and Technology

This course will cover the basic theory of Lie groups and Lie algebras. The prequisites include knowledge of linear algebra and group theory as covered by Algebra courses and basic notions of differential geometry (manifolds, vector fields,… etc).


  • 13:30 – 16:00, Monday, Wednesday and Thursday at Lecture hall 301A, Building A5.

  • The first lecture will be held on Wednesday, October 29, 2008.


Professor Pierre Cartier, IHES

The best way to contact Professor P. Cartier is during the lecture or at his office (room 110, building A5)





  • Introduction: Global and infinitesimal symmetris

  • Lie algebras: Basic definitions, enveloping algebra, Hopf lgebras, classical Lie algebras, Cartan subalgebras (roots and weights)

  • Lie groups: Classical Lie groups, Lie algebra of a Lie group, algebraic groups, maximal torus and Bruhat decomposition

  • Basic results about linear representations

  • A glimpse into modern developments: Quantum groups , Lie groupoids





  1. A. Kirillov Jr., Introduction to Lie Groups and Lie Algebras, Cambridge University Press, 2002

  2. N. Bourbaki, Lie groups and Lie algebras Chapter 1-3 ISBN 3-540-64242-0, Chapters 4-6 ISBN 3-540-42650-7, Chapters 7-9 ISBN 3-540-43405-4

  3. J. P. Serre, Lie Algebras and Lie Groups: 1964 Lectures given at Harvard University, LNM 1500, Springer

  4. R. Carter et al., Lectures on Lie Groups and Lie Algebras, LMS Student Texts Series, 1995

  5. J. E. Humphreys, Introduction to Lie Algebras and Representaion theory, Springer 1978

Note: Almost all of these textbooks are available at the library of the Institute of Mathematics. Some of them are available electronically also.


The final exam will be posted here.

P.S. Cái này mình copy trên trang của Viện Toán, bác nào rảnh thì đi nghe nhá!