What is quaternion?

Quaternion

In mathematics, the quaternions are a number system that extends the complex numbers. They were first described by Irish mathematician William Rowan Hamilton in 1843 and applied to mechanics in three-dimensional space. A feature of quaternions is that the product of two quaternions is noncommutative. Hamilton defined a quaternion as the quotient of two directed lines in a three-dimensional space or equivalently as the quotient of two vectors. Quaternions can also be represented as the sum of a scalar and a vector.

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Some articles on quaternion:

Classical Hamiltonian Quaternions - Classical Elements of A Quaternion - Quaternion - Right Quaternion
... A right quaternion is a quaternion whose scalar component is zero, The angle of a right quaternion is 90 degrees ... A right quaternion can also be thought of as a vector plus a zero scalar ... Right quaternions may be put in what was called the standard trinomial form ...
Quaternion - Quotes
... I regard it as an inelegance, or imperfection, in quaternions, or rather in the state to which it has been hitherto unfolded, whenever it becomes or seems to become necessary to have recourse to x ... The mathematical quaternion partakes of both these elements in technical language it may be said to be "time plus space", or "space plus time" and in this sense it has, or at least involves a ... Quaternions came from Hamilton after his really good work had been done and, though beautifully ingenious, have been an unmixed evil to those who have touched them in any way ...
Classical Hamiltonian Quaternions - Other Operators in Detail - Tensor
... Symbolically A quaternion is by definition the quotient of two vectors and the tensor of a quaternion is by definition the quotient of the tensors of these two vectors ... In symbols From this definition it can be shown that a useful formula for the tensor of a quaternion is It can also be proven from this definition that another ... The square root of the common norm of a quaternion is equal to its tensor A useful identity is that the square of the tensor of a quaternion is equal to the tensor of the square of a ...
Versor
... In modern algebra, a versor or unit quaternion is a quaternion of norm one ... When used to represent a rotation, a versor will rotate any quaternion vector v through the angle θ around the unit vector r through the sandwiching product qvq ... by William Rowan Hamilton, in the context of his quaternion theory ...
Classical Hamiltonian Quaternions - Other Operators in Detail - Axis and Angle
... Taking the angle of a non-scalar quaternion, resulted in a value greater than zero and less than π ... When a non-scalar quaternion is viewed as the quotient of two vectors, then the axis of the quaternion is a unit vector perpendicular to the plane of the two ...