Split-biquaternion

In mathematics, a split-biquaternion is a hypercomplex number of the form
where "w", "x", "y", and "z" are split-complex numbers and i, j, and k multiply as in the quaternion group. Since each coefficient "w", "x", "y", "z" spans two real dimensions, the split-biquaternion is an element of an eight-dimensional vector space. Considering that it carries a multiplication, this vector space is an algebra over the real field, or an algebra over a ring where the split-complex numbers form the ring. This algebra was introduced by William Kingdon Clifford in an 1873 article for the London Mathematical Society. It has been repeatedly noted in mathematical literature since then, variously as a deviation in terminology, an illustration of the tensor product of algebras, and as an illustration of the direct sum of algebras.
The split-biquaternions have been identified in various ways by algebraists; see below.
Modern definition.
A split-biquaternion is ring isomorphic to the Clifford algebra "C"ℓ0,3(R). This is the geometric algebra generated by three orthogonal imaginary unit basis directions, under the combination rule
giving an algebra spanned by the 8 basis elements with ("e"1"e"2)2 = ("e"2"e"3)2 = ("e"3"e"1)2 = −1 and ω2 = ("e"1"e"2"e"3)2 = +1.
The sub-algebra spanned by the 4 elements is the division ring of Hamilton's quaternions, .
One can therefore see that
where is the algebra spanned by the algebra of the split-complex numbers.
Equivalently,
Split-biquaternion group.
The split-biquaternions form an associative ring as is clear from considering multiplications in its basis {1, ω, i, j, k, ωi, ωj, ωk}. When ω is adjoined to the quaternion group one obtains a 16 element group 
Direct sum of two quaternion rings.
The direct sum of the division ring of quaternions with itself is denoted formula_5. The product of two elements formula_6 and formula_7 is formula_8 in this direct sum algebra.
Proposition: The algebra of split-biquaternions is isomorphic to formula_9
proof: Every split-biquaternion has an expression "q" = "w" + "z" ω where "w" and "z" are quaternions and ω2 = +1. Now if "p" = "u" + "v" ω is another split-biquaternion, their product is 
The isomorphism mapping from split-biquaternions to formula_5 is given by
In formula_5, the product of these images, according to the algebra-product of formula_5 indicated above, is
This element is also the image of pq under the mapping into formula_9
Thus the products agree, the mapping is a homomorphism; and since it is bijective, it is an isomorphism.
Though split-biquaternions form an eight-dimensional space like Hamilton's biquaternions, on the basis of the Proposition it is apparent that this algebra splits into the direct sum of two copies of the real quaternions.
Hamilton biquaternion.
The split-biquaternions should not be confused with the (ordinary) biquaternions previously introduced by William Rowan Hamilton. Hamilton's biquaternions are elements of the algebra
Synonyms.
The following terms and compounds refer to the split-biquaternion algebra: