# Klein four-group

In mathematics, the Klein four-group (or just Klein group or Vierergruppe (English: four-group), often symbolized by the letter V) is the group Z2 × Z2, the direct product of two copies of the cyclic group of order 2. It was named Vierergruppe by Felix Klein in his Vorlesungen über das Ikosaeder und die Auflösung der Gleichungen vom fünften Grade (Lectures on the icosahedron and the solution of equations of the fifth degree) in 1884.

The Klein four-group is the smallest non-cyclic group. It is given by the group presentation

$\mathrm{V}_4 = \langle a,b \mid a^2 = b^2 = (ab)^2 = 1 \rangle.$

All non-identity elements of the Klein group have order 2. It is abelian, and isomorphic to the dihedral group of order (cardinality) 4. It is also isomorphic to the direct sum Z2 ⊕ Z2, so that it can be represented as the bit strings {00, 01, 10, 11} under bitwise XOR.

The Klein group's Cayley table is given by:

* 1 a b ab
1 1 a b ab
a a 1 ab b
b b ab 1 a
ab ab b a 1

An elementary construction of the Klein four-group is the multiplicative group { 1, 3, 5, 7 } with the action being multiplication modulo 8. Here a is 3, b is 5, and ab is 3 × 5 = 15 ≡ 7 (mod 8).

In 2D it is the symmetry group of a rhombus and of a rectangle which are not squares, the four elements being the identity, the vertical reflection, the horizontal reflection, and a 180 degree rotation.

The symmetry group of this cross is the Klein four-group. It can be flipped horizontally or vertically and remain unchanged. Unlike a square, though, a quarter-turn rotation will change the figure.

In 3D there are three different symmetry groups that are algebraically the Klein four-group V:

• one with three perpendicular 2-fold rotation axes: D2
• one with a 2-fold rotation axis, and a perpendicular plane of reflection: C2h = D1d
• one with a 2-fold rotation axis in a plane of reflection (and hence also in a perpendicular plane of reflection): C2v = D1h.

The three elements of order 2 in the Klein four-group are interchangeable: the automorphism group is the group of permutations of the three elements. This group's permutations of its own elements can be thought of abstractly as its permutation representation on 4 points:

Identity and double-transpositions of four objects form V
Other permutations of four objects, forming V as well

See: 4 element subsets of S4
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. In fact, it is the kernel of a surjective map from S4 to S3. According to Galois theory, the existence of the Klein four-group (and in particular, this representation of it) explains the existence of the formula for calculating the roots of quartic equations in terms of radicals, as established by Lodovico Ferrari: the map S4 → S3 corresponds to the resolvent cubic, in terms of Lagrange resolvents.

The Klein four-group as a subgroup of A4 is not the automorphism group of any simple graph. It is, however, the automorphism group of a two-vertex graph where the vertices are connected to each other with two edges, making the graph non-simple. It is also the automorphism group of the following simple graph, but in the permutation representation { (), (1,2), (3,4), (1,2)(3,4) } where the points are labeled top-left, bottom-left, top-right, bottom-right:

In the construction of finite rings, eight of the eleven rings with four elements have the Klein four-group as their additive substructure.

In music composition the four-group is the basic group of permutations in the twelve-tone technique. In that instance the Cayley table is written;[1]

 S I: R: RI: I: S RI R R: RI S I RI: R I S

In interval analysis the set of intervals [a,b] on the real line is identified with the points (a,b) of the Cartesian plane with Euclidean distance. The Klein four-group arises from the multiplicative structure on the direct sum RR. The x-axis and the y-axis are ideals of the ring of intervals where [1, 1] is the identity so quadrant I is the component of the identity. The Klein four-group is the quotient of the multiplicative group of intervals that do not end on an axis by the subgroup consisting of intervals represented by points in quadrant I. Similarly, the ring of intervals is normalized to the split-complex number ring which also produces the Klein four-group as the quotient of its group of units by its identity component.