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The (2,3,7) triangle group admits a presentation in terms of the group of quaternions of norm 1 in a suitable order in a quaternion algebra.More specifically, the triangle group is the quotient of the group of quaternions by its center ±1.
The triangle group is the infinite symmetry group of a certain tessellation (or tiling) of the Euclidean plane by triangles whose angles add up to π (or 180°). Up to permutations, the triple (l, m, n) is one of the triples (2,3,6), (2,4,4), (3,3,3). The corresponding triangle groups are instances of wallpaper groups.
The dihedral group D 3 is the symmetry group of an equilateral triangle, that is, it is the set of all rigid transformations (reflections, rotations, and combinations of these) that leave the shape and position of this triangle fixed. In the case of D 3, every possible permutation of the triangle's vertices constitutes such a transformation, so ...
All of the discrete point symmetries are subgroups of certain continuous symmetries. They can be classified as products of orthogonal groups O( n ) or special orthogonal groups SO( n ). O(1) is a single orthogonal reflection, dihedral symmetry order 2, Dih 1 .
A co-compact example is the (ordinary, rotational) (2,3,7) triangle group, containing the Fuchsian groups of the Klein quartic and of the Macbeath surface, as well as other Hurwitz groups. More generally, any hyperbolic von Dyck group (the index 2 subgroup of a triangle group, corresponding to orientation-preserving isometries) is a Fuchsian group.
Moreover, two objects are considered to be of the same symmetry type if their symmetry groups are conjugate subgroups of O(3) (two subgroups H 1, H 2 of a group G are conjugate, if there exists g ∈ G such that H 1 = g −1 H 2 g). For example, two 3D objects have the same symmetry type:
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.
If H and K are subgroups of a group G, the commutator of H and K, denoted by [H, K], is defined as the subgroup of G generated by commutators between elements in the two subgroups. If L is a third subgroup, the convention that [H,K,L] = [[H,K],L] will be followed.
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