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.
(Reference: G. Baumslag and D. Solitar, Some two generator one-relator non-Hopfian groups, Bull. Amer. Math. Soc. , 689 (1962) pp. 199–201.)
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}>
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>
(Reference: W. Magnus, Braid groups: A survey, Proceedings of the Second International Conference on the Theory of Groups, Canberra, Australia, 1973, pp. 463-487.)
Cyclic groups
C(n) = <a | a^{n} = 1>
Dicyclic groups
Dic(n) = <a,b | a^{2n} = 1, a^{n} = b^{2}, bab^{-1} = a^{-1}>
Dihedral groups
The dihedral group of degree n is the group of symmetries of a regular polynom. This non-abelian group consists of 2n elements, n rotations and n reflections. Let r be a single rotation and s be an arbitrary reflection. Then the group has the following representation
Dih(n) = <r,s | r^{n} = s^{2} = (rs)^{2} = 1>
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>
Free abelian groups
Z(n) = <a_{1},...,a_{n} | [a_{i},a_{j}] = 1 for all i,j>
Free groups
F(n) = <a_{1},...,a_{n} | a_{i}a_{i}^{-1} = a_{i}^{-1}a_{i} = 1>
Fibonacci groups
Fibonacci groups are related to the inductive definition of the Fibonacci numbers f_{i} + f_{i+1} = f_{i+2} where f_{1} = f_{2} = 1. These groups have the following finite presentation:
F(2,m) = <x_{1},...,x_{m} | x_{i}x_{i+1} = x_{i+2}>
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>
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 tetrahedon groups
The Ordinary tetrahedon 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>
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
