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# Klein four-group

In mathematics, the Klein four-group (or just Klein group or Vierergruppe, often symbolized by the letter V), named after Felix Klein, is a group with four elements, the smallest non-cyclic group.

Its multiplication table[?] is given by:

1abc
11abc
aa1cb
bbc1a
ccba1

It may be visualized as the symmetry group of a rectangle:

```    *************
*           *
*************
```

the four elements being: the identity, the vertical reflection, the horizontal reflection, and a 180 degree rotation.

All elements of the Klein group (except the identity) have order 2. It is abelian, and is isomorphic to C2 × C2, the direct product of two copies of the cyclic group of order 2. It is also isomorphic to the dihedral group of order 4.

The essential symmetry between the three elements of order 2 in the Klein four-group can be seen by its permutation representation[?] on 4 points:

V = < (1,2)(3,4), (1,3)(2,4), (1,4)(2,3) >

In this representation, V is a normal subgroup of the alternating group A4 (and also the symmetric group S4) on 4 letters. According to Galois theory, the existence of the Klein four-group (and in particular, this particular representation) explains the existence of the formula for calculating the roots of quartic equations in terms of radicals.

One can also think of the Klein four-group as the automorphism group of the following graph:

```   *   *
|   |
*   *
|
*
```

All Wikipedia text is available under the terms of the GNU Free Documentation License

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