Tag Archives: group-theory

Isomorphism of factors does not imply isomorphism of quotient groups

Let G be a group and H,K two isomorphic subgroups. We provide an example where the quotient groups G/H and G/K are not isomorphic.

Let G=Z4×Z2, with H=(¯2,¯0) and K=(¯0,¯1). We have HKZ2. The left cosets of H in G are G/H={(¯0,¯0)+H,(¯1,¯0)+H,(¯0,¯1)+H,(¯1,¯1)+H}, a group having 4 elements and for all elements xG/H, one can verify that 2x=H. Hence G/HZ2×Z2. The left cosets of K in G are G/K={(¯0,¯0)+K,(¯1,¯0)+K,(¯2,¯0)+K,(¯3,¯0)+K}, which is a cyclic group of order 4 isomorphic to Z4. We finally get the desired conclusion G/HZ2×Z2Z4G/K.

A group that is not a semi-direct product

Given a group G with identity element e, a subgroup H, and a normal subgroup NG; then we say that G is the semi-direct product of N and H (written G=NH) if G is the product of subgroups, G=NH where the subgroups have trivial intersection NH={e}.

Semi-direct products of groups provide examples of non abelian groups. For example the dihedral group D2n with 2n elements is isomorphic to a semidirect product of the cyclic groups Zn and Z2. D2n is the group of isometries preserving a regular polygon X with n edges.

Let’see that the converse is not true and present a group that is not a semi-direct product.

The Hamilton’s quaternions group is not a semi-direct product

The Hamilton’s quaternions group H8 is the group consisting of the symbols ±1,±i,±j,±k where1=i2=j2=k2 and ij=k=ji,jk=i=kj,ki=j=ik. One can prove that H8 endowed with the product operation above is indeed a group having 8 elements where 1 is the identity element.

H8 is not abelian as ij=kk=ji.

Let’s prove that H8 is not the semi-direct product of two subgroups. If that was the case, there would exist a normal subgroup N and a subgroup H such that G=NH.

  • If |N|=4 then H={1,h} where h is an element of order 2 in H8. Therefore h=1 which is the only element of order 2. But 1N as 1 is the square of all elements in H8{±1}. We get the contradiction NH{1}.
  • If |N|=2 then |H|=4 and H is also normal in G. Noting N={1,n} we have for hH h1nh=n and therefore nh=hn. This proves that the product G=NH is direct. Also N is abelian as a cyclic group of order 2. H is also cyclic as all groups of order p2 with p prime are abelian. Finally G would be abelian, again a contradiction.

We can conclude that G is not a semi-direct product.

A normal subgroup that is not a characteristic

Let’s G be a group. A characteristic subgroup is a subgroup HG that is mapped to itself by every automorphism of G.

An inner automorphism is an automorphism φAut(G) defined by a formula φ:xa1xa where a is an element of G. An automorphism of a group which is not inner is called an outer automorphism. And a subgroup HG that is mapped to itself by every inner automorphism of G is called a normal subgroup.

Obviously a characteristic subgroup is a normal subgroup. The converse is not true as we’ll see below.

Example of a direct product

Let K be a nontrivial group. Then consider the group G=K×K. The subgroups K1={e}×K and K2=K×{e} are both normal in G as for (e,k)K1 and (a,b)G we have
(a,b)1(e,x)(a,b)=(a1,b1)(e,x)(a,b)=(e,b1xb)K1 and b1K1b=K1. Similar relations hold for K2. As K is supposed to be nontrivial, we have K1K2.

The exchange automorphism ψ:(x,y)(y,x) exchanges the subgroup K1 and K2. Thus, neither K1 nor K2 is invariant under all the automorphisms, so neither is characteristic. Therefore, K1 and K2 are both normal subgroups of G that are not characteristic.

When K=Z2 is the cyclic group of order two, G=Z2×Z2 is the Klein four-group. In particular, this gives a counterexample where the ambient group is an abelian group.

Example on the additive group Q

Consider the additive group (Q,+) of rational numbers. The map φ:xx/2 is an automorphism. As (Q,+) is abelian, all subgroups are normal. However, the subgroup Z is not sent into itself by φ as φ(1)=1/2Z. Hence Z is not a characteristic subgroup.

Is the quotient group of a finite group always isomorphic to a subgroup?

Given a normal subgroup H of a finite group G, is G/H always isomorphic to a subgroup KG?

The case of an abelian group

According to the fundamental theorem of finite abelian groups, every finite abelian group G can be expressed as the direct sum of cyclic subgroups of prime-power order: Gui=1Zpαii where p1,,pu are primes and α1,,αu non zero integers.

If HG we have Hui=1Zpβii with 0β1α1,,0βuαu. Then G/Hui=1Zpαiβii which is a subgroup of G.

If G is not abelian, then G/H might not be isomorphic to a subgroup of G. Continue reading Is the quotient group of a finite group always isomorphic to a subgroup?

A simple group whose order is not a prime

Consider a finite group G whose order (number of elements) is a prime number. It is well known that G is cyclic and simple. Which means that G has no non trivial normal subgroup.

Is the converse true, i.e. are the cyclic groups with prime orders the only simple groups? The answer is negative. We prove here that for n5 the alternating group An is simple. In particular A5 whose order is equal to 60 is simple. Continue reading A simple group whose order is not a prime

An infinite group whose proper subgroups are all finite

We study some properties of the Prüfer p-group Zp for a prime number p. The Prüfer p-group may be identified with the subgroup of the circle group, consisting of all pn-th roots of unity as n ranges over all non-negative integers:
Zp=k=0Zpk where Zpk={e2iπmpk | 0mpk1}

Zp is a group

First, let’s notice that for 0mn integers we have ZpmZpn as pm|pn. Also for m0 Zpm is a subgroup of the circle group. We also notice that all elements of Zp have finite orders which are powers of p. Continue reading An infinite group whose proper subgroups are all finite

The set of all commutators in a group need not be a subgroup

I here provide a simple example of a group whose set of commutators is not a subgroup. The counterexample is due to P.J. Cassidy (1979).

Description of the group G

Let k[x,y] denote the ring of all polynomials in two variables over a field k, and let k[x] and k[y] denote the subrings of all polynomials in x and in y respectively. G is the set of all upper unitriangular matrices of the form
A=(1f(x)h(x,y)01g(y)001) where f(x)k[x], g(y)k[y], and h(x,y)k[x,y]. The matrix A will also be denoted (f,g,h).
Let’s verify that G is a group. The products of two elements (f,g,h) and (f,g,h) is
(1f(x)h(x,y)01g(y)001)(1f(x)h(x,y)01g(y)001)
=(1f(x)+f(x)h(x,y)+h(x,y)+f(x)g(y)01g(y)+g(y)001) which is an element of G. We also have:
(1f(x)h(x,y)01g(y)001)1=(1f(x)f(x)g(y)h(x,y)01g(y)001) proving that the inverse of an element of G is also an element of G. Continue reading The set of all commutators in a group need not be a subgroup

Two subgroups whose product is not a subgroup

In this article, we consider a group G and two subgroups H and K. Let HK={hk | hH,kK}.

HK is a subgroup of G if and only if HK=KH. For the proof we first notice that if HK is a subgroup of G then it’s closed under inverses so HK=(HK)1=K1H1=KH. Conversely if HK=KH then take hk, hkHK. Then (hk)(hk)1=hk(k)1(h)1. Since HK=KH we can rewrite k(k)1(h)1 as hk for some new hH, kK. So (hk)(hk)1=hhk which is in HK. This verifies that HK is a subgroup. Continue reading Two subgroups whose product is not a subgroup

A finitely generated soluble group isomorphic to a proper quotient group

Let Q2 be the ring of rational numbers of the form m2n with m,nZ and N=U(3,Q2) the group of unitriangular matrices of dimension 3 over Q2. Let t be the diagonal matrix with diagonal entries: 1,2,1 and put H=t,N. We will prove that H is finitely generated and that one of its quotient group G is isomorphic to a proper quotient group of G. Continue reading A finitely generated soluble group isomorphic to a proper quotient group

Converse of Lagrange’s theorem does not hold

Lagrange’s theorem, states that for any finite group G, the order (number of elements) of every subgroup H of G divides the order of G (denoted by |G|).

Lagrange’s theorem raises the converse question as to whether every divisor d of the order of a group is the order of some subgroup. According to Cauchy’s theorem this is true when d is a prime.

However, this does not hold in general: given a finite group G and a divisor d of |G|, there does not necessarily exist a subgroup of G with order d. The alternating group G=A4, which has 12 elements has no subgroup of order 6. We prove it below. Continue reading Converse of Lagrange’s theorem does not hold