# Group Examples

### Non-abelian Groups

#### Baumslag groups

The Baumslag (respectively Baumslag-Solitar) groups are examples of two-generator one-relator groups. The first variation of this group has the presentation

```B(m,n) = <a, b | a^m = b^n = 1>
```

where m, n are natural numbers. The second variation of this group, known as the Baumslag-Solitar group, has the presentation

```BS(m,n) = <a, b | b*a^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 Braid group has n strands and n - 1 generators:

```B(n) = <g_1, g_2, ... , 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>
```

Type Braid(n, [DegreeBound, LoopBound]) to calculate the Gröbner base.

(Reference: W. Magnus, Braid groups: A survey, Proceedings of the Second International Conference on the Theory of Groups, Canberra, Australia, 1973, pp. 463-487.)

#### Dihedral groups

The Dihedral group of degree n (denoted by Dih_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>
```

#### 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_nx_k = x_{n+1} for all k < n>
with a = x_0, x_n = a^{1-n}ba^{n-1}
```

#### 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>
```

#### 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>
```

#### Free abelian group

```Z^n = <a_1,...,a_n | [a_i,a_j] = 1 for all i,j>
```

#### Free group

```F(n) = <a,b | aa^{-1} = a^{-1}a = bb^{-1} = b^{-1}b = 1>
```

#### Cyclic group

```C_n = <a | a^n = 1>
```

#### Dicyclic group

```DC_n = <a,b | a^{2n} = 1,a^n = b^2, bab^{-1} = a^{-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}>
```

#### Fibonacci groups

The 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}>
```

#### Ordinary Tetrahedon groups

The Ordinary Tetrahedon groups are groups with the following representation where e_i >= 2 and fi >= 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>
```

#### 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
```