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Each rotation is represented by two unit quaternions of opposite sign, and, as in the space of rotations in three dimensions, the quaternion product of two unit quaternions will yield a unit quaternion. Also, the space of unit quaternions is "flat" in any infinitesimal neighborhood of a given unit quaternion.
For example, the equation z 2 + 1 = 0, has infinitely many quaternion solutions, which are the quaternions z = b i + c j + d k such that b 2 + c 2 + d 2 = 1. Thus these "roots of –1" form a unit sphere in the three-dimensional space of vector quaternions.
Since unit quaternions are subject to two algebraic constraints, unit quaternions are standard to represent rigid transformations. [2] Similar to the way that rotations in 3D space can be represented by quaternions of unit length, rigid motions in 3D space can be represented by dual quaternions of unit length.
Like rotation matrices, quaternions must sometimes be renormalized due to rounding errors, to make sure that they correspond to valid rotations. The computational cost of renormalizing a quaternion, however, is much less than for normalizing a 3 × 3 matrix. Quaternions also capture the spinorial character of rotations in three dimensions.
A direct formula for the conversion from a quaternion to Euler angles in any of the 12 possible sequences exists. [2] For the rest of this section, the formula for the sequence Body 3-2-1 will be shown. If the quaternion is properly normalized, the Euler angles can be obtained from the quaternions via the relations:
The reciprocal of a unit vector is the vector reversed. [44] = = Because a unit vector and its reciprocal are parallel to each other but point in opposite directions, the product of a unit vector and its reciprocal have a special case commutative property, for example if a is any unit vector then: [45]
It can also be realized as the subgroup of unit quaternions generated by [10] = / and =. The generalized quaternion groups have the property that every abelian subgroup is cyclic. [ 11 ] It can be shown that a finite p -group with this property (every abelian subgroup is cyclic) is either cyclic or a generalized quaternion group as defined ...
The Lipschitz quaternions L form an index 2 sublattice of H. The group of units in L is the order 8 quaternion group Q = {±1, ±i, ±j, ±k}. The group of units in H is a nonabelian group of order 24 known as the binary tetrahedral group. The elements of this group include the 8 elements of Q along with the 16 quaternions {(±1 ± i ± j ± k ...