Euclidean group

The number of degrees of freedom for E(n) is n(n + 1)/2, which gives 3 in case n = 2, and 6 for n = 3. Of these, n can be attributed to available translational symmetry, and the remaining n(n − 1)/2 to rotational symmetry.

The direct isometries (i.e., isometries preserving the handedness of chiral subsets) comprise a subgroup of E(n), called the special Euclidean group and usually denoted by E⁺(n) or SE(n). They include the translations and rotations, and combinations thereof; including the identity transformation, but excluding any reflections.

The isometries that reverse handedness are called indirect, or opposite. For any fixed indirect isometry R, such as a reflection about some hyperplane, every other indirect isometry can be obtained by the composition of R with some direct isometry. Therefore, the indirect isometries are a coset of E⁺(n), which can be denoted by E⁻(n). It follows that the subgroup E⁺(n) is of index 2 in E(n).

The natural topology of Euclidean space ${\displaystyle \mathbb {E} ^{n}}$ implies a topology for the Euclidean group E(n). Namely, a sequence f_i of isometries of ${\displaystyle \mathbb {E} ^{n}}$ (${\displaystyle i\in \mathbb {N} }$) is defined to converge if and only if, for any point p of ${\displaystyle \mathbb {E} ^{n}}$, the sequence of points p_i converges.

From this definition it follows that a function ${\displaystyle f:[0,1]\to E(n)}$ is continuous if and only if, for any point p of ${\displaystyle \mathbb {E} ^{n}}$, the function ${\displaystyle f_{p}:[0,1]\to \mathbb {E} ^{n}}$ defined by f_p(t) = (f(t))(p) is continuous. Such a function is called a "continuous trajectory" in E(n).

It turns out that the special Euclidean group SE(n) = E⁺(n) is connected in this topology. That is, given any two direct isometries A and B of ${\displaystyle \mathbb {E} ^{n}}$, there is a continuous trajectory f in E⁺(n) such that f(0) = A and f(1) = B. The same is true for the indirect isometries E⁻(n). On the other hand, the group E(n) as a whole is not connected: there is no continuous trajectory that starts in E⁺(n) and ends in E⁻(n).

The continuous trajectories in E(3) play an important role in classical mechanics, because they describe the physically possible movements of a rigid body in three-dimensional space over time. One takes f(0) to be the identity transformation I of ${\displaystyle \mathbb {E} ^{3}}$, which describes the initial position of the body. The position and orientation of the body at any later time t will be described by the transformation f(t). Since f(0) = I is in E⁺(3), the same must be true of f(t) for any later time. For that reason, the direct Euclidean isometries are also called "rigid motions".

The Euclidean groups are not only topological groups, they are Lie groups, so that calculus notions can be adapted immediately to this setting.

The Euclidean group E(n) is a subgroup of the affine group for n dimensions, and in such a way as to respect the semidirect product structure of both groups. This gives, a fortiori, two ways of writing elements in an explicit notation. These are:

1. by a pair (A, b), with A an n × n orthogonal matrix, and b a real column vector of size n; or
2. by a single square matrix of size n + 1, as explained for the affine group.

Details for the first representation are given in the next section.

In the terms of Felix Klein's Erlangen programme, we read off from this that Euclidean geometry, the geometry of the Euclidean group of symmetries, is, therefore, a specialisation of affine geometry. All affine theorems apply. The origin of Euclidean geometry allows definition of the notion of distance, from which angle can then be deduced.

The Euclidean group is a subgroup of the group of affine transformations.

It has as subgroups the translational group T(n), and the orthogonal group O(n). Any element of E(n) is a translation followed by an orthogonal transformation (the linear part of the isometry), in a unique way:

$${\displaystyle x\mapsto A(x+b)}$$

where A is an orthogonal matrix or the same orthogonal transformation followed by a translation:

$${\displaystyle x\mapsto Ax+c,}$$

with c = Ab T(n) is a normal subgroup of E(n): for every translation t and every isometry u, the composition

$${\displaystyle u^{-1}tu}$$

is again a translation. Together, these facts imply that E(n) is the semidirect product of O(n) extended by T(n), which is written as ${\displaystyle {\text{E}}(n)={\text{T}}(n)\rtimes {\text{O}}(n)}$. In other words, O(n) is (in the natural way) also the quotient group of E(n) by T(n):

$${\displaystyle {\text{O}}(n)\cong {\text{E}}(n)/{\text{T}}(n)}$$

Now SO(n), the special orthogonal group, is a subgroup of O(n) of index two. Therefore, E(n) has a subgroup E⁺(n), also of index two, consisting of direct isometries. In these cases the determinant of A is 1.

They are represented as a translation followed by a rotation, rather than a translation followed by some kind of reflection (in dimensions 2 and 3, these are the familiar reflections in a mirror line or plane, which may be taken to include the origin, or in 3D, a rotoreflection).

This relation is commonly written as:

$${\displaystyle {\text{SO}}(n)\cong {\text{E}}^{+}(n)/{\text{T}}(n)}$$

or, equivalently:

$${\displaystyle {\text{E}}^{+}(n)={\text{SO}}(n)\ltimes {\text{T}}(n).}$$

Types of subgroups of E(n):

Finite groups.

They always have a fixed point. In 3D, for every point there are for every orientation two which are maximal (with respect to inclusion) among the finite groups: O_h and I_h. The groups I_h are even maximal among the groups including the next category.

Countably infinite groups without arbitrarily small translations, rotations, or combinations

i.e., for every point the set of images under the isometries is topologically discrete (e.g., for 1 ≤ m ≤ n a group generated by m translations in independent directions, and possibly a finite point group). This includes lattices. Examples more general than those are the discrete space groups.

Countably infinite groups with arbitrarily small translations, rotations, or combinations

In this case there are points for which the set of images under the isometries is not closed. Examples of such groups are, in 1D, the group generated by a translation of 1 and one of √2, and, in 2D, the group generated by a rotation about the origin by 1 radian.

Non-countable groups, where there are points for which the set of images under the isometries is not closed

(e.g., in 2D all translations in one direction, and all translations by rational distances in another direction).

Non-countable groups, where for all points the set of images under the isometries is closed

e.g.:

• all direct isometries that keep the origin fixed, or more generally, some point (in 3D called the rotation group)
• all isometries that keep the origin fixed, or more generally, some point (the orthogonal group)
• all direct isometries E⁺(n)
• the whole Euclidean group E(n)
• one of these groups in an m-dimensional subspace combined with a discrete group of isometries in the orthogonal (n−m)-dimensional space
• one of these groups in an m-dimensional subspace combined with another one in the orthogonal (n−m)-dimensional space

Examples in 3D of combinations:

• all rotations about one fixed axis
• ditto combined with reflection in planes through the axis and/or a plane perpendicular to the axis
• ditto combined with discrete translation along the axis or with all isometries along the axis
• a discrete point group, frieze group, or wallpaper group in a plane, combined with any symmetry group in the perpendicular direction
• all isometries which are a combination of a rotation about some axis and a proportional translation along the axis; in general this is combined with k-fold rotational isometries about the same axis (k ≥ 1); the set of images of a point under the isometries is a k-fold helix; in addition there may be a 2-fold rotation about a perpendicularly intersecting axis, and hence a k-fold helix of such axes.
• for any point group: the group of all isometries which are a combination of an isometry in the point group and a translation; for example, in the case of the group generated by inversion in the origin: the group of all translations and inversion in all points; this is the generalized dihedral group of R³, Dih(R³).

E(1), E(2), and E(3) can be categorized as follows, with degrees of freedom:

Chasles' theorem asserts that any element of E⁺(3) is a screw displacement.

See also 3D isometries that leave the origin fixed, space group, involution.

For some isometry pairs composition does not depend on order:

• two translations
• two rotations or screws about the same axis
• reflection with respect to a plane, and a translation in that plane, a rotation about an axis perpendicular to the plane, or a reflection with respect to a perpendicular plane
• glide reflection with respect to a plane, and a translation in that plane
• inversion in a point and any isometry keeping the point fixed
• rotation by 180° about an axis and reflection in a plane through that axis
• rotation by 180° about an axis and rotation by 180° about a perpendicular axis (results in rotation by 180° about the axis perpendicular to both)
• two rotoreflections about the same axis, with respect to the same plane
• two glide reflections with respect to the same plane

The translations by a given distance in any direction form a conjugacy class; the translation group is the union of those for all distances.

In 1D, all reflections are in the same class.

In 2D, rotations by the same angle in either direction are in the same class. Glide reflections with translation by the same distance are in the same class.

In 3D:

• Inversions with respect to all points are in the same class.
• Rotations by the same angle are in the same class.
• Rotations about an axis combined with translation along that axis are in the same class if the angle is the same and the translation distance is the same.
• Reflections in a plane are in the same class
• Reflections in a plane combined with translation in that plane by the same distance are in the same class.
• Rotations about an axis by the same angle not equal to 180°, combined with reflection in a plane perpendicular to that axis, are in the same class.