Hyperoctahedral group

In mathematics, a hyperoctahedral group is an important type of group that can be realized as the group of symmetries of a hypercube or of a cross-polytope. It was named by Alfred Young in 1930. Groups of this type are identified by a parameter , the dimension of the hypercube.

As a Coxeter group it is of type , and as a Weyl group it is associated to the symplectic groups and with the orthogonal groups in odd dimensions. As a wreath product it is $S_2 \wr S_n$ where is the symmetric group of degree . As a permutation group, the group is the signed symmetric group of permutations ''π'' either of the set or of the set such that for all . As a matrix group, it can be described as the group of orthogonal matrices whose entries are all integers. Equivalently, this is the set of matrices with entries only 0, 1, or –1, which are invertible, and which have exactly one non-zero entry in each row or column. The representation theory of the hyperoctahedral group was described by according to .

In three dimensions, the hyperoctahedral group is known as where is the octahedral group, and is a symmetric group (here a cyclic group) of order 2. Geometric figures in three dimensions with this symmetry group are said to have octahedral symmetry, named after the regular octahedron, or 3-orthoplex. In 4-dimensions it is called a hexadecachoric symmetry, after the regular 16-cell, or 4-orthoplex. In two dimensions, the hyperoctahedral group structure is the abstract dihedral group of order eight, describing the symmetry of a square, or 2-orthoplex.

By dimension

Hyperoctahedral groups can be named as B_n, a bracket notation, or as a Coxeter group graph:

Subgroups

There is a notable index two subgroup, corresponding to the Coxeter group ''D''_''n'' and the symmetries of the demihypercube. Viewed as a wreath product, there are two natural maps from the hyperoctahedral group to the cyclic group of order 2: one map coming from "multiply the signs of all the elements" (in the ''n'' copies of $\$), and one map coming from the parity of the permutation. Multiplying these together yields a third map $C_n \to \$. The kernel of the first map is the Coxeter group $D_n.$ In terms of signed permutations, thought of as matrices, this third map is simply the determinant, while the first two correspond to "multiplying the non-zero entries" and "parity of the underlying (unsigned) permutation", which are not generally meaningful for matrices, but are in the case due to the coincidence with a wreath product.

The kernels of these three maps are all three index two subgroups of the hyperoctahedral group, as discussed in ''H''₁: Abelianization below, and their intersection is the derived subgroup, of index 4 (quotient the Klein 4-group), which corresponds to the rotational symmetries of the demihypercube.

In the other direction, the center is the subgroup of scalar matrices, ; geometrically, quotienting out by this corresponds to passing to the projective orthogonal group.

In dimension 2 these groups completely describe the hyperoctahedral group, which is the dihedral group Dih₄ of order 8, and is an extension 2.V (of the 4-group by a cyclic group of order 2). In general, passing to the subquotient (derived subgroup, mod center) is the symmetry group of the projective demihypercube.

The hyperoctahedral subgroup, D_n by dimension:

The chiral hyper-octahedral symmetry, is the direct subgroup, index 2 of hyper-octahedral symmetry.

Another notable index 2 subgroup can be called hyper-pyritohedral symmetry, by dimension: These groups have ''n'' orthogonal mirrors in ''n''-dimensions.

Homology

The group homology of the hyperoctahedral group is similar to that of the symmetric group, and exhibits stabilization, in the sense of stable homotopy theory.

H₁: abelianization

The first homology group, which agrees with the abelianization, stabilizes at the Klein four-group, and is given by:
:$H_1(C_n, \mathbf) = \begin 0 & n = 0\\ \mathbf/2 & n = 1\\ \mathbf/2 \times \mathbf/2 & n \geq 2 \end.$
This is easily seen directly: the $-1$ elements are order 2 (which is non-empty for $n\geq 1$), and all conjugate, as are the transpositions in $S_n$ (which is non-empty for $n\geq 2$), and these are two separate classes. These elements generate the group, so the only non-trivial abelianizations are to 2-groups, and either of these classes can be sent independently to $-1 \in \,$ as they are two separate classes. The maps are explicitly given as "the product of the signs of all the elements" (in the ''n'' copies of $\$), and the sign of the permutation. Multiplying these together yields a third non-trivial map (the determinant of the matrix, which sends both these classes to $-1$), and together with the trivial map these form the 4-group.

H₂: Schur multipliers

The second homology groups, known classically as the Schur multipliers, were computed in .

They are:
:$H_2(C_n,\mathbf) = \begin 0 & n = 0, 1\\ \mathbf/2 & n = 2\\ (\mathbf/2)^2 & n = 3\\ (\mathbf/2)^3 & n \geq 4 \end.$

Notes

References

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Finite reflection groups