### Table of Contents

## Groups

Abbreviation: **Grp**

### Definition

A \emph{group} is a structure $\mathbf{G}=\langle G,\cdot ,^{-1},e\rangle $, where $\cdot $ is an infix binary operation, called the \emph{group product}, $^{-1}$ is a postfix unary operation, called the \emph{group inverse} and $e$ is a constant (nullary operation), called the \emph{identity element}, such that

$\cdot $ is associative: $(xy)z=x(yz)$

$e$ is a left-identity for $\cdot$: $ex=x$

$^{-1}$ gives a left-inverse: $x^{-1}x=e$.

Remark: It follows that $e$ is a right-identity and that $^{-1}$gives a right inverse: $xe=x$, $xx^{-1}=e$.

##### Morphisms

Let $\mathbf{G}$ and $\mathbf{H}$ be groups. A morphism from $\mathbf{G}$ to $\mathbf{H}$ is a function $h:Garrow H$ that is a homomorphism:

$h(xy)=h(x)h(y)$, $h(x^{-1})=h(x)^{-1}$, $h(e)=e$

### Examples

Example 1: $\langle S_{X},\circ ,^{-1},id_{X}\rangle $, the collection of permutations of a sets $X$, with composition, inverse, and identity map.

Example 2: The general linear group $\langle GL_{n}(V),\cdot ,^{-1},I_{n}\rangle $, the collection of invertible $n\times n$ matrices over a vector space $V$, with matrix multiplication, inverse, and identity matrix.

### Basic results

### Properties

Classtype | variety |
---|---|

Equational theory | decidable in polynomial time |

Quasiequational theory | undecidable |

First-order theory | undecidable |

Congruence distributive | no ($\mathbb{Z}_{2}\times \mathbb{Z}_{2}$) |

Congruence modular | yes |

Congruence n-permutable | yes, n=2, $p(x,y,z)=xy^{-1}z$ is a Mal'cev term |

Congruence regular | yes |

Congruence uniform | yes |

Congruence types | 1=permutational |

Congruence extension property | no, consider a non-simple subgroup of a simple group |

Definable principal congruences | |

Equationally def. pr. cong. | no |

Amalgamation property | yes |

Strong amalgamation property | yes |

Epimorphisms are surjective | yes |

Locally finite | no |

Residual size | unbounded |

### Finite members

$\begin{array}{lr}
f(1)= &1

f(2)= &1

f(3)= &1

f(4)= &2

f(5)= &1

f(6)= &2

f(7)= &1

f(8)= &5

f(9)= &2

f(10)= &2

f(11)= &1

f(12)= &5

f(13)= &1

f(14)= &2

f(15)= &1

f(16)= &14

f(17)= &1

f(18)= &5

\end{array}$

Information about small groups up to size 2000: http://www.tu-bs.de/~hubesche/small.html