Group Examples
Non-abelian Groups
Baumslag groups
Baumslag-Solitar groups are examples of two-generator one-relator groups.
BS(m,n) = <a,b | ba^{m} = a^{n}b>
where m, n are natural numbers. Computations of Baumslag groups.
Another variation of the Baumslag groups, called the Baumslag-Gersten group, is defined by:
BG = <a,b | (b^{-1}a^{-1}b)a(b^{-1}a^{-1}b) = a^{2}>
Computations of Baumslag-Gersten groups.
Braid groups
For a natural number n, the following is a presentation of the Braid group with n-1 generators and n strands.
B(n) = <g_{1},...,g_{n-1} | g_{i}g_{j} = g_{j}g_{i} for |i-j| >= 2, g_{i}g_{i+1}g_{i} = g_{i+1}g_{i}g_{i+1} for 1 <= i <= n-2>
Artin groups (Generalized Braid groups)
An Artin group (Generalized Braid group) is a group with a representation of the form
G = <x_{1},...,x_{n} | <x_1,x_2>^{m_{1,2}} = <x_2,x_1>^{m_{2,1}}, ... , <x_{n-1},x_{n}>^{m_{n-1,n}} = <x_{n},x_{n-1}>^{m_{n,n-1}}>
where m_{i,j} = m_{j,i} > 1
If m is not infinity <x_i,x_j>^{m} denotes an alterning product of x_i and x_j of length m beginning with x_i.
For example <x_1,x_2>^{4} = x_1x_2_x1_x2
If m is infinity there is no relation between x_i and x_j.
Cyclic groups
For a natural number n > 1 the cyclic groups can be represented as
C(n) = <a | a^{n} = 1>
Dicyclic groups
Dic(n) = <a,b | a^{2n} = 1, a^{n} = b^{2}, b^{-1}ab = a^{-1}>
Computations of dicyclic groups
Dihedral groups
Dih(n) = <r,s | r^{n} = s^{2} = s^{-1}rs = r^{-1} = 1>
Computations of dihedral groups
von Dyck groups
The von Dyck groups are sometimes referred to as ordinary triangle groups and are subgroups of index 2 in Triangle(l, m, n) generated by words of even length in the generators a, b, c. A specific representation is given for x = ab, y = ca, yx = cb:
D(l,m,n) = <x,y | x^{l} = y^{m} = (xy)^{n} = 1>
Computations of von Dyck groups
Free abelian groups
Z(n) = <a_{1},...,a_{n} | [a_{i},a_{j}] = 1 for all i,j>
Computations of free abelian groups
Free groups
F(n) = <a_{1},...,a_{n} | a_{i}a_{i}^{-1} = a_{i}^{-1}a_{i} = 1>
Fibonacci groups
These groups have the following finite presentation:
F(2,m) = <x_{1},...,x_{m} | x_{i}x_{i+1} = x_{i+2}>
where the indices are taken modulo m. Computations of Fibonacci groups
Hecke groups
H(lambda_q) = <x,y | x^2=(xy)^q=1, for q >= 3>
Heisenberg groups
H(2k+1) = <a_{1},...,a_{k},b_{1},...,b_{k},c | [a_{i},b_{i}] = c, [a_{i},c] = [b_{i},c], [a_{i},b_{j}] = 1 for all i != j>
Computations of Heisenberg groups
Higman group
H = <a,b,c,d | a^{-1}ba = b^{2}, b^{-1}cb = c^{2}, c^{-1}dc = d^{2}, d^{-1}ad = a^{2}>
Ordinary tetrahedron groups
The Ordinary tetrahedron groups are groups with the following presentation where e_i >= 2 and f_i >= 2 for all i.
G(e_1,e_2,e_3,f_1,f_2,f_3) = <x,y,z | x^{e_1} = y^{e_2} = z^{e_3} = (xy^{-1})^{f_1} = (yz^{-1})^{f_2} = (zx^{-1})^{f_3} = 1>
Computations of Ordinary Tetrahedron groups
Lamplighter group
The Lamplighter group has the following presentation
G = <a,b | (ab^{n}ab^{-n})^2 = 1>
for all n in Z. Computations of Lamplighter group
Symmetric groups
The symmetric groups have the following presentation for n > 0
S_n = <a_{1},..,a_{n-1} | a_{i}^2 = 1, a_{i}a_{j} = a_{j}a_{i} for j != i +/- 1, (a_{i}a_{i+1})^3 = 1>
where a_{i} is the neighbor transposition a_{i} = (i,i+1) Computations of symmetric groups
Tetraeder group
The Tetraeder group has the following presentation
A_4 = <a,b | a^2 = b^3 = (ab)^3 = 1>
Computations of the Tetraeder group
Oktaeder group
The Oktaeder group has the following presentation
O = <a,b | a^2 = b^3 = (ab)^4 = 1>
Computations of the Oktaeder group
Ikosaeder group
The Ikosaeder group has the following presentation
I = <a,b | a^2 = b^3 = (ab)^5 = 1>
Computations of the Ikosaeder group
Mathieu group M_{11}
The Mathieu group M_{22} has the following presentation
M_{11} = <x,y | xy = y^{-3}x^{-4}, y[x^{-1},y^{-1}] = [x^{-1},y]x^{-1}>
Computations of the Mathieu group M_{11} References: EFFICIENT PRESENTATIONS FOR THREE SIMPLE GROUPS P.E. Kenne Department of Computer Science, Australian National University, GPO Box 4, Canberra ACT 2601.
Mathieu group M_{22}
The Mathieu group M_{22} has the following presentation
M_{22} = <a,b | a^2 = (ab)^{11}, (ababb)^7 = b^4, (ab)^{2}(ab^{-1})^{2}abb(ab)^{2}ab^{-1}ab(abb)^2=b>
Computations of the Mathieu group M_{22} References: Efficient presentations for the Mathieu simple group M 22 and its cover Marston Conder, George Havas and Colin Ramsay
Quaternion group
The Quaternion group has the following presentation
Q_8 = <a,b | a^4 = 1, x^2 = y^2, y^{-1}xy = x^{-1}>
Computations of the Quaternion group
Tits group
The Tits group has the following presentation
T = <a,b | a^2 = b^3 = (ab)^13 = [a,b]^5 = [a,bab]^4 = ((ab)^4ab^{-1})^6 = 1 >
Computations of the Tits group
Special Linear group
The special linear group has the following presentation
SL_2(Z) = <a,b | aba = bab, (aba)^4 = 1 >
Computations of the Special Linear group
Special Linear group SL_2(32)
The special linear group S_2(32) has the following presentation
SL_2(32) = <a,b | b^3 = (ab)^{2} = a^{6}ba^{-2}ba^{2}b^{-1}a^{3}b^{-1}ab^{-1}a^{-3}b >
Computations of the Special Linear group SL_2(32) Reference: EFFICIENT PRESENTATIONS FOR THREE SIMPLE GROUPS P.E. Kenne Department of Computer Science, Australian National University, GPO Box 4, Canberra ACT 2601.
Special Linear group with prime p
The Special Linear group SL_2(p) has the following presentation
SL_2(p) = (x,y | x^{2} =(xy)^{3},(xy^{4}xy^{t})^{2}y^{p}x^{2k}=1>
Computations of the Special Linear group with prime p Reference: EFFICIENT PRESENTATIONS FOR FINITE SIMPLE GROUPS AND RELATED GROUPS Colin M. Campbell, E.F. Robertson and P.D. Williams* 1988
Modular group
The Modular group has the following presentation
PSL(2,Z) = <a,b | a^2 = (ab)^3 = 1 >
Computations of the Modular group
Alternating groups
The Alternating groups have the following presentation
A_{n+2} = <x_{1},..x_{n} | x_{i}^{3} = (x_{i}x_{j})^2 = 1 for every i != j>
Computations of the Alternating groups
Thompson group
T = <a,b | [ab^{-1},a^{-1}ba] = [ab^{-1},a^{-2}ba^{2}] = 1>
= <x_{0},x_{1},x_{2},... | x_{k}^{-1}x_{n}x_{k} = x_{n+1} for all k < n> with a = x_{0}, x_{n} = a^{1-n}ba^{n-1}
Triangle groups
The triangle groups describe the application of reflections across the sides of a triangle (A,B,C) with the three reflections a,b,c and can be represented as the following:
Triangle(l,m,n) = {a,b,c | a^{2} = b^{2} = c^{2} = (ab)^{l} = (bc)^{m} = (ca)^{n} = 1}
There are three different cases depending on the choice of the parameters l,m,n:
The euclidian case: 1/l + 1/m + 1/n = 1 The spherical case: 1/l + 1/m + 1/n > 1 The hyperbolical case: 1/l + 1/m + 1/n < 1
Computations of the Triangle groups
Other groups
The following groups are either special cases of the groups above or hasn't been named yet.
The first example is a group which has the same count of generators and relations (#Generators = #Relations) and is solvable with length 6.
1) G = <a,b | a^{2}b^{-6} = (ab^{-1})^{3}ab^{-2}ab^{k}a^{-1}b = 1>
where k is congruent to 3 mod 6.
Computations of the other group 1
The next group, denoted by G, is the largest finite generalized triangle group and called the Rosenberger Monster. This group has an order |G| = 2^{20}*3^{4}*5.
2) G = <a,b | a^2 = b^3 = (abababab^{2}ab^{2}abab^{2}ab^{2})^2 = 1>
Levai, Rosenberger and Souvignier showed that G is finite an the group below, denoted by H, is infinite.
3) H = <a,b | a^2 = b^3 = (abababab^{2}abab^{2}ab^{2})^2 = 1>
Computations of the other groups 2 and 3
This group is an example of a generalized triangle group with order |G| = 1440
4) G = <a,b | a^2 = b^3 = (ababab^{2})^{3} = 1>
Computations of the other group 4
The following groups, denoted by G and H, are both solvable with length 4 but differs in the factors. G has an order |G| = 4224.
5) G = <a,b | a^{2}b^{-4} = (ababab^{3})^{2} = 1>
6) H = <a,b | a^{2}b^{4} = (ababab^{3})^{2} = 1>
Computations of the other groups 5 and 6
The next group denoted by G has the following representation:
7) G = <a,b | a^{2}b^{-3} = (ababa^{2}ab^{2})^2 = 1>
It holds that |G| = 9216 and that G is solvable of length 4.
Computations of the other group 7
The next group, denoted by H(r,n,s) has more relations and depends on three parameters. The finite representation for r > s and s >= 0 is given by:
8) H(r,n,s) = <a_{1},a_{2},...,a_{n} | a_{1}a_{2}..a_{r} = a_{r+1}a_{r+2}..a_{r+s}, a_{2}a_{3}..a_{r+1} =
a_{r+2}a_{r+3}..a_{r+s+1},..,a_{n}a_{1}..a_{r-1} = a_{r}a_{r+1}..a_{r+s-1}>
Computations of the other group 8
This group has the following finite representation:
9) F(r,n,k) = <a_{1},a_{2},..,a_{n} | a_{1}a_{2}..a_{r} = a_{r+k},a_{2}a_{3}..a_{r+1} = a_{r+k+1},..,a_{n}a_{1}a_{2}..a_{r-1} =
a_{r+k-1}>
It follow that the group number 8 (denoted by H(r,n,s)) is isomorphic to the group F(r,n,k) for s = k = 1.
A special case which is also isomorphic to H(r,n,s) is the group denoted by F(r,n). A finite representation is given by:
10) F(r,n) = <a_{1},..,a_{n} | a_{1}a_{2}..a_{r} = a_{r+1},..,a_{n}a_{1}..a_{r-1} = a_{r}>
Computations of the other groups 9 and 10
The next group consists of two generators and two relations (#generators = #relations) with the following representation:
11) G = <x,t | xt^{r} = tx^{r},t^{n} = 1>
for r >= 1 and n >= 2
Computations of the other group 11
Group number 12 has the following finite representation:
12) G = <x,t | tx^{a}t^{-1} = x^{b},t^{n} = 1>
for a,b >= 1 and n >= 2. In this case (like group number 11 above) the count of generators is equal to the count of relations.
Computations of the other group 12
For the next group we need four parameters: a,b,c,d. It is represented by this representation:
13) G = <x,y | x^2 = xy^{a}xy^{b}xy^{c}xy^{d} = 1>
Computations of the other group 13
