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Definition

Let {1, i, j, k} be the basis for the (real) quaternions H, and let u, v, w, x be complex numbers, then

q = u 1 + v i + w j + x k {\displaystyle q=u\mathbf {1} +v\mathbf {i} +w\mathbf {j} +x\mathbf {k} }

is a biquaternion.⁶ To distinguish square roots of minus one in the biquaternions, Hamilton⁷⁸ and Arthur W. Conway used the convention of representing the square root of minus one in the scalar field C by h to avoid confusion with the i in the quaternion group. Commutativity of the scalar field with the quaternion group is assumed:

h i = i h ,     h j = j h ,     h k = k h . {\displaystyle h\mathbf {i} =\mathbf {i} h,\ \ h\mathbf {j} =\mathbf {j} h,\ \ h\mathbf {k} =\mathbf {k} h.}

Hamilton introduced the terms bivector, biconjugate, bitensor, and biversor to extend notions used with real quaternions H.

Hamilton's primary exposition on biquaternions came in 1853 in his Lectures on Quaternions. The editions of Elements of Quaternions, in 1866 by William Edwin Hamilton (son of Rowan), and in 1899, 1901 by Charles Jasper Joly, reduced the biquaternion coverage in favour of the real quaternions.

Considered with the operations of component-wise addition, and multiplication according to the quaternion group, this collection forms a 4-dimensional algebra over the complex numbers C. The algebra of biquaternions is associative, but not commutative. A biquaternion is either a unit or a zero divisor. The algebra of biquaternions forms a composition algebra and can be constructed from bicomplex numbers. See § As a composition algebra below.

Place in ring theory

Linear representation

Note that the matrix product

( h 0 0 − h ) ( 0 1 − 1 0 ) = ( 0 h h 0 ) {\displaystyle {\begin{pmatrix}h&0\\0&-h\end{pmatrix}}{\begin{pmatrix}0&1\\-1&0\end{pmatrix}}={\begin{pmatrix}0&h\\h&0\end{pmatrix}}} .

Because h is the imaginary unit, each of these three arrays has a square equal to the negative of the identity matrix. When this matrix product is interpreted as i j = k, then one obtains a subgroup of matrices that is isomorphic to the quaternion group. Consequently,

( u + h v w + h x − w + h x u − h v ) {\displaystyle {\begin{pmatrix}u+hv&w+hx\\-w+hx&u-hv\end{pmatrix}}}

represents biquaternion q = u 1 + v i + w j + x k. Given any 2 × 2 complex matrix, there are complex values u, v, w, and x to put it in this form so that the matrix ring M(2, C) is isomorphic⁹ to the biquaternion ring. This representation also shows that the 16-element group

{ ± 1 , ± h , ± i , ± h i , ± j , ± h j , ± k , ± h k } {\displaystyle \{\pm \mathbf {1} ,\pm h,\pm \mathbf {i} ,\pm h\mathbf {i} ,\pm \mathbf {j} ,\pm h\mathbf {j} ,\pm \mathbf {k} ,\pm h\mathbf {k} \}}

is isomorphic to the Pauli group, the central product of a cyclic group of order 4 and the dihedral group of order 8. Concretely, the Pauli matrices

X = ( 0 1 1 0 ) , Y = ( 0 − h h 0 ) , Z = ( 1 0 0 − 1 ) {\displaystyle X={\begin{pmatrix}0&1\\1&0\end{pmatrix}},\quad Y={\begin{pmatrix}0&-h\\h&0\end{pmatrix}},\quad Z={\begin{pmatrix}1&0\\0&-1\end{pmatrix}}}

correspond respectively to the elements －hk, －hj, and －hi.

Subalgebras

Considering the biquaternion algebra over the scalar field of real numbers R, the set

{ 1 , h , i , h i , j , h j , k , h k } {\displaystyle \{\mathbf {1} ,h,\mathbf {i} ,h\mathbf {i} ,\mathbf {j} ,h\mathbf {j} ,\mathbf {k} ,h\mathbf {k} \}}

forms a basis so the algebra has eight real dimensions. The squares of the elements hi, hj, and hk are all positive one, for example, (hi)2 = h2i2 = (−1)(−1) = +1.

The subalgebra given by

{ x + y ( h i ) : x , y ∈ R } {\displaystyle \{x+y(h\mathbf {i} ):x,y\in \mathbb {R} \}}

is ring isomorphic to the plane of split-complex numbers, which has an algebraic structure built upon the unit hyperbola. The elements hj and hk also determine such subalgebras.

Furthermore,

{ x + y j : x , y ∈ C } {\displaystyle \{x+y\mathbf {j} :x,y\in \mathbb {C} \}}

is a subalgebra isomorphic to the bicomplex numbers.

A third subalgebra called coquaternions is generated by hj and hk. It is seen that (hj)(hk) = (−1)i, and that the square of this element is −1. These elements generate the dihedral group of the square. The linear subspace with basis {1, i, hj, hk} thus is closed under multiplication, and forms the coquaternion algebra.

In the context of quantum mechanics and spinor algebra, the biquaternions hi, hj, and hk (or their negatives), viewed in the M2(C) representation, are called Pauli matrices.

Algebraic properties

The biquaternions have two conjugations:

• the biconjugate or biscalar minus bivector is q ∗ = w − x i − y j − z k   , {\displaystyle q^{*}=w-x\mathbf {i} -y\mathbf {j} -z\mathbf {k} \!\ ,} and
• the complex conjugation of biquaternion coefficients q ¯ = w ¯ + x ¯ i + y ¯ j + z ¯ k {\displaystyle {\bar {q}}={\bar {w}}+{\bar {x}}\mathbf {i} +{\bar {y}}\mathbf {j} +{\bar {z}}\mathbf {k} }

where z ¯ = a − b h {\displaystyle {\bar {z}}=a-bh} when z = a + b h , a , b ∈ R , h 2 = − 1 . {\displaystyle z=a+bh,\quad a,b\in \mathbb {R} ,\quad h^{2}=-\mathbf {1} .}

Note that ( p q ) ∗ = q ∗ p ∗ , p q ¯ = p ¯ q ¯ , q ∗ ¯ = q ¯ ∗ . {\displaystyle (pq)^{*}=q^{*}p^{*},\quad {\overline {pq}}={\bar {p}}{\bar {q}},\quad {\overline {q^{*}}}={\bar {q}}^{*}.}

Clearly, if q q ∗ = 0 {\displaystyle qq^{*}=0} then q is a zero divisor. Otherwise { q q ∗ } − 1 {\displaystyle \lbrace qq^{*}\rbrace ^{-\mathbf {1} }} is a complex number. Further, q q ∗ = q ∗ q {\displaystyle qq^{*}=q^{*}q} is easily verified. This allows the inverse to be defined by

• q − 1 = q ∗ { q q ∗ } − 1 {\displaystyle q^{-1}=q^{*}\lbrace qq^{*}\rbrace ^{-1}} , if q q ∗ ≠ 0. {\displaystyle qq^{*}\neq 0.}

Let q = w + v {\displaystyle q=w\,+\,\mathbf {v} } where v = x i + y j + z k {\displaystyle \mathbf {v} =x\,\mathbf {i} +y\,\mathbf {j} +z\,\mathbf {k} } . The square v 2 {\displaystyle \mathbf {v} ^{2}} is a complex number. Any entire function's power series of q {\displaystyle q} will sum to c + d v {\displaystyle c+d\,\mathbf {v} } for some complex c and d.

Non-unique square roots and logarithms of biquaternions exist except for exceptional cases. They are found by solving ( a + b v ) 2 = q {\displaystyle (a+b\,\mathbf {v} )^{2}=q\,\,} and exp ( a + b v ) = q {\displaystyle {\text{exp}}(a+b\,\mathbf {v} )=q} , respectively, for complex a and b. Other solutions may be found by multiplying (square root) or by adding (logarithm) c + d v {\displaystyle c+d\,\mathbf {v} } , where the complex c and d satisfy ( c + d v ) 2 = 1 {\displaystyle (c+d\,\mathbf {v} )^{2}=1} (square root) and exp ( c + d v ) = 1 {\displaystyle {\text{exp}}(c+d\,\mathbf {v} )=1} (logarithm). If the v part of q {\displaystyle q} is non-zero and has square zero, the square root does not generally exist. If v has square zero, log ( v ) {\displaystyle {\text{log}}(\mathbf {v} )} does not exist.

Relation to Lorentz transformations

Further information: Lorentz transformation via quaternions and hyperbolic numbers and Relativistic biquaternions by Noether (1910), Klein (1910), Conway (1911), Silberstein (1911)

Consider now the linear subspace[10]

M = { q : q ∗ = q ¯ } = { t + x ( h i ) + y ( h j ) + z ( h k ) : t , x , y , z ∈ R } . {\displaystyle M=\lbrace q\colon q^{*}={\bar {q}}\rbrace =\lbrace t+x(h\mathbf {i} )+y(h\mathbf {j} )+z(h\mathbf {k} )\colon t,x,y,z\in \mathbb {R} \rbrace .}

M is not a subalgebra since it is not closed under products; for example ( h i ) ( h j ) = h 2 i j = − k ∉ M . {\displaystyle (h\mathbf {i} )(h\mathbf {j} )=h^{2}\mathbf {ij} =-\mathbf {k} \notin M.} Indeed, M cannot form an algebra if it is not even a magma.

Proposition: If q is in M, then q q ∗ = t 2 − x 2 − y 2 − z 2 . {\displaystyle qq^{*}=t^{2}-x^{2}-y^{2}-z^{2}.}

Proof: From the definitions,

q q ∗ = ( t + x h i + y h j + z h k ) ( t − x h i − y h j − z h k ) = t 2 − x 2 ( h i ) 2 − y 2 ( h j ) 2 − z 2 ( h k ) 2 = t 2 − x 2 − y 2 − z 2 . {\displaystyle {\begin{aligned}qq^{*}&=(t+xh\mathbf {i} +yh\mathbf {j} +zh\mathbf {k} )(t-xh\mathbf {i} -yh\mathbf {j} -zh\mathbf {k} )\\&=t^{2}-x^{2}(h\mathbf {i} )^{2}-y^{2}(h\mathbf {j} )^{2}-z^{2}(h\mathbf {k} )^{2}\\&=t^{2}-x^{2}-y^{2}-z^{2}.\end{aligned}}}

Definition: Let biquaternion g satisfy g g ∗ = 1. {\displaystyle gg^{*}=1.} Then the Lorentz transformation associated with g is given by

T ( q ) = g ∗ q g ¯ . {\displaystyle T(q)=g^{*}q{\bar {g}}.}

Proposition: If q is in M, then T(q) is also in M.

Proof: ( g ∗ q g ¯ ) ∗ = g ¯ ∗ q ∗ g = g ∗ ¯ q ¯ g = g ∗ q g ¯ ) ¯ . {\displaystyle (g^{*}q{\bar {g}})^{*}={\bar {g}}^{*}q^{*}g={\overline {g^{*}}}{\bar {q}}g={\overline {g^{*}q{\bar {g}})}}.}

Proposition: T ( q ) ( T ( q ) ) ∗ = q q ∗ {\displaystyle \quad T(q)(T(q))^{*}=qq^{*}}

Proof: Note first that gg* = 1 implies that the sum of the squares of its four complex components is one. Then the sum of the squares of the complex conjugates of these components is also one. Therefore, g ¯ ( g ¯ ) ∗ = 1. {\displaystyle {\bar {g}}({\bar {g}})^{*}=1.} Now

( g ∗ q g ¯ ) ( g ∗ q g ¯ ) ∗ = g ∗ q ( g ¯ g ¯ ∗ ) q ∗ g = g ∗ q q ∗ g = q q ∗ . {\displaystyle (g^{*}q{\bar {g}})(g^{*}q{\bar {g}})^{*}=g^{*}q({\bar {g}}{\bar {g}}^{*})q^{*}g=g^{*}qq^{*}g=qq^{*}.}

Associated terminology

As the biquaternions have been a fixture of linear algebra since the beginnings of mathematical physics, there is an array of concepts that are illustrated or represented by biquaternion algebra. The transformation group G = { g : g g ∗ = 1 } {\displaystyle G=\lbrace g:gg^{*}=1\rbrace } has two parts, G ∩ H {\displaystyle G\cap H} and G ∩ M . {\displaystyle G\cap M.} The first part is characterized by g = g ¯ {\displaystyle g={\bar {g}}}  ; then the Lorentz transformation corresponding to g is given by T ( q ) = g − 1 q g {\displaystyle T(q)=g^{-1}qg} since g ∗ = g − 1 . {\displaystyle g^{*}=g^{-1}.} Such a transformation is a rotation by quaternion multiplication, and the collection of them is SO(3) ≅ G ∩ H . {\displaystyle \cong G\cap H.} But this subgroup of G is not a normal subgroup, so no quotient group can be formed.

To view G ∩ M {\displaystyle G\cap M} it is necessary to show some subalgebra structure in the biquaternions. Let r represent an element of the sphere of square roots of minus one in the real quaternion subalgebra H. Then (hr)2 = +1 and the plane of biquaternions given by D r = { z = x + y h r : x , y ∈ R } {\displaystyle D_{r}=\lbrace z=x+yhr:x,y\in \mathbb {R} \rbrace } is a commutative subalgebra isomorphic to the plane of split-complex numbers. Just as the ordinary complex plane has a unit circle, D r {\displaystyle D_{r}} has a unit hyperbola given by

exp ⁡ ( a h r ) = cosh ⁡ ( a ) + h r   sinh ⁡ ( a ) , a ∈ R . {\displaystyle \exp(ahr)=\cosh(a)+hr\ \sinh(a),\quad a\in R.}

Just as the unit circle turns by multiplication through one of its elements, so the hyperbola turns because exp ⁡ ( a h r ) exp ⁡ ( b h r ) = exp ⁡ ( ( a + b ) h r ) . {\displaystyle \exp(ahr)\exp(bhr)=\exp((a+b)hr).} Hence these algebraic operators on the hyperbola are called hyperbolic versors. The unit circle in C and unit hyperbola in Dr are examples of one-parameter groups. For every square root r of minus one in H, there is a one-parameter group in the biquaternions given by G ∩ D r . {\displaystyle G\cap D_{r}.}

The space of biquaternions has a natural topology through the Euclidean metric on 8-space. With respect to this topology, G is a topological group. Moreover, it has analytic structure making it a six-parameter Lie group. Consider the subspace of bivectors A = { q : q ∗ = − q } {\displaystyle A=\lbrace q:q^{*}=-q\rbrace } . Then the exponential map exp : A → G {\displaystyle \exp :A\to G} takes the real vectors to G ∩ H {\displaystyle G\cap H} and the h-vectors to G ∩ M . {\displaystyle G\cap M.} When equipped with the commutator, A forms the Lie algebra of G. Thus this study of a six-dimensional space serves to introduce the general concepts of Lie theory. When viewed in the matrix representation, G is called the special linear group SL(2,C) in M(2, C).

Many of the concepts of special relativity are illustrated through the biquaternion structures laid out. The subspace M corresponds to Minkowski space, with the four coordinates giving the time and space locations of events in a resting frame of reference. Any hyperbolic versor exp(ahr) corresponds to a velocity in direction r of speed c tanh a where c is the velocity of light. The inertial frame of reference of this velocity can be made the resting frame by applying the Lorentz boost T given by g = exp(0.5ahr) since then g ⋆ = exp ⁡ ( − 0.5 a h r ) = g ∗ {\displaystyle g^{\star }=\exp(-0.5ahr)=g^{*}} so that T ( exp ⁡ ( a h r ) ) = 1. {\displaystyle T(\exp(ahr))=1.} Naturally the hyperboloid G ∩ M , {\displaystyle G\cap M,} which represents the range of velocities for sub-luminal motion, is of physical interest. There has been considerable work associating this "velocity space" with the hyperboloid model of hyperbolic geometry. In special relativity, the hyperbolic angle parameter of a hyperbolic versor is called rapidity. Thus we see the biquaternion group G provides a group representation for the Lorentz group.¹⁰

After the introduction of spinor theory, particularly in the hands of Wolfgang Pauli and Élie Cartan, the biquaternion representation of the Lorentz group was superseded. The new methods were founded on basis vectors in the set

{ q   :   q q ∗ = 0 } = { w + x i + y j + z k   :   w 2 + x 2 + y 2 + z 2 = 0 } {\displaystyle \{q\ :\ qq^{*}=0\}=\left\{w+x\mathbf {i} +y\mathbf {j} +z\mathbf {k} \ :\ w^{2}+x^{2}+y^{2}+z^{2}=0\right\}}

which is called the complex light cone. The above representation of the Lorentz group coincides with what physicists refer to as four-vectors. Beyond four-vectors, the standard model of particle physics also includes other Lorentz representations, known as scalars, and the (1, 0) ⊕ (0, 1)-representation associated with e.g. the electromagnetic field tensor. Furthermore, particle physics makes use of the SL(2, C) representations (or projective representations of the Lorentz group) known as left- and right-handed Weyl spinors, Majorana spinors, and Dirac spinors. It is known that each of these seven representations can be constructed as invariant subspaces within the biquaternions.¹¹

As a composition algebra

Although W. R. Hamilton introduced biquaternions in the 19th century, its delineation of its mathematical structure as a special type of algebra over a field was accomplished in the 20th century: the biquaternions may be generated out of the bicomplex numbers in the same way that Adrian Albert generated the real quaternions out of complex numbers in the so-called Cayley–Dickson construction. In this construction, a bicomplex number (w, z) has conjugate (w, z)* = (w, – z).

The biquaternion is then a pair of bicomplex numbers (a, b), where the product with a second biquaternion (c, d) is

( a , b ) ( c , d ) = ( a c − d ∗ b , d a + b c ∗ ) . {\displaystyle (a,b)(c,d)=(ac-d^{*}b,da+bc^{*}).}

If a = ( u , v ) , b = ( w , z ) , {\displaystyle a=(u,v),b=(w,z),} then the biconjugate ( a , b ) ∗ = ( a ∗ , − b ) . {\displaystyle (a,b)^{*}=(a^{*},-b).}

When (a, b)* is written as a 4-vector of ordinary complex numbers,

( u , v , w , z ) ∗ = ( u , − v , − w , − z ) . {\displaystyle (u,v,w,z)^{*}=(u,-v,-w,-z).}

The biquaternions form an example of a quaternion algebra, and it has norm

N ( u , v , w , z ) = u 2 + v 2 + w 2 + z 2 . {\displaystyle N(u,v,w,z)=u^{2}+v^{2}+w^{2}+z^{2}.}

Two biquaternions p and q satisfy N(pq) = N(p) N(q), indicating that N is a quadratic form admitting composition, so that the biquaternions form a composition algebra.

See also

• Biquaternion algebra
• Hypercomplex number
• Hypercomplex analysis
• Complex quaternion functions
• Joachim Lambek
• MacFarlane's use
• Quotient ring

Citations

The Wikibook Associative Composition Algebra has a page on the topic of: Biquaternions

• Arthur Buchheim (1885) "A Memoir on biquaternions", American Journal of Mathematics 7(4):293 to 326 from Jstor early content.
• Conway, Arthur W. (1911), "On the application of quaternions to some recent developments in electrical theory", Proceedings of the Royal Irish Academy, 29A: 1–9.
• Dickson, Leonard (1914), Linear Algebras, §13 "Equivalence of the complex quaternion and matric algebras" via HathiTrust
• Furey, C. (2012). "Unified Theory of Ideals". Phys. Rev. D. 86 (2): 025024. arXiv:1002.1497. Bibcode:2012PhRvD..86b5024F. doi:10.1103/PhysRevD.86.025024. S2CID 118458623.
• Francis; Kosowsky (2005), "The construction of spinors in geometric algebra", Annals of Physics, 317 (2): 317, 384–409, arXiv:math-ph/0403040, Bibcode:2005AnPhy.317..383F, doi:10.1016/j.aop.2004.11.008, S2CID 119632876
• Garling, D. J. H. (2011), Clifford Algebras: An Introduction, Cambridge University Press
• Girard, P. R. (1984), "The quaternion group and modern physics", European Journal of Physics, 5 (1): 25–32, Bibcode:1984EJPh....5...25G, doi:10.1088/0143-0807/5/1/007, S2CID 250775753
• Hamilton, Willian R. (1850), "On Geometrical Interpretation of Some Results obtained by Calculation with Biquaternions", Proceedings of the Royal Irish Academy, 5: 388
• Hamilton, William Rowan (1853), Lectures on Quaternions, Article 669, Hodges and Smith; [etc., etc.]
• Hamilton, William Rowan (1866), Hamilton, William Edwin (ed.), Elements of Quaternions (1st ed.), Longmans, Green & Co.
• Hamilton, William Rowan (1899), Joly, Jasper Joly (ed.), Elements of Quaternions, vol. I (2nd ed.), Longmans, Green & Co.
• Hamilton, William Rowan (1901), Joly, Jasper Joly (ed.), Elements of Quaternions, vol. II (2nd ed.), Longmans, Green & Co.
• Hermann, Robert (1974), Spinors, Clifford and Cayley Algebras, Interdisciplinary Mathematics, vol. VII, Math Sci Press, pp. 250–265, ISBN 0-915692-06-6
• Kilmister, C. W. (1994), Eddington's search for a fundamental theory, Cambridge University Press, pp. 121, 122, 179, 180, ISBN 978-0-521-37165-0
• Kravchenko, Vladislav (2003), Applied Quaternionic Analysis, Heldermann Verlag, ISBN 3-88538-228-8
• Lanczos, Cornelius (1949), The Variational Principles of Mechanics, University of Toronto Press, pp. 304–312
• Silberstein, Ludwik (1912), "Quaternionic form of relativity", Philosophical Magazine, Series 6, 23 (137): 790–809, doi:10.1080/14786440508637276
• Silberstein, Ludwik (1914), The Theory of Relativity
• Synge, J. L. (1972), "Quaternions, Lorentz transformations, and the Conway-Dirac-Eddington matrices", Communications of the Dublin Institute for Advanced Studies, Series A, 21
• Sangwine, Stephen J.; Ell, Todd A.; Le Bihan, Nicolas (2010), "Fundamental representations and algebraic properties of biquaternions or complexified quaternions", Advances in Applied Clifford Algebras, 21 (3): 1–30, arXiv:1001.0240, doi:10.1007/s00006-010-0263-3, S2CID 54729224
• Sangwine, Stephen J.; Alfsmann, Daniel (2010), "Determination of the biquaternion divisors of zero, including idempotents and nilpotents", Advances in Applied Clifford Algebras, 20 (2): 401–410, arXiv:0812.1102, Bibcode:2008arXiv0812.1102S, doi:10.1007/s00006-010-0202-3, S2CID 14246706
• Tanişli, M. (2006), "Gauge transformation and electromagnetism with biquaternions", Europhysics Letters, 74 (4): 569, Bibcode:2006EL.....74..569T, doi:10.1209/epl/i2005-10571-6, S2CID 250862773

References

1. Hamilton 1850. - Hamilton, Willian R. (1850), "On Geometrical Interpretation of Some Results obtained by Calculation with Biquaternions", Proceedings of the Royal Irish Academy, 5: 388 https://books.google.com/books?id=ggoFAAAAQAAJ&dq=proceedings+of+royal+irish+academy+1844+Hamilton&pg=PA388 ↩

2. Garling 2011, pp. 112, 113. - Garling, D. J. H. (2011), Clifford Algebras: An Introduction, Cambridge University Press ↩

3. Garling 2011, p. 112. - Garling, D. J. H. (2011), Clifford Algebras: An Introduction, Cambridge University Press ↩

4. Francis & Kosowsky 2005, p. 404. - Francis; Kosowsky (2005), "The construction of spinors in geometric algebra", Annals of Physics, 317 (2): 317, 384–409, arXiv:math-ph/0403040, Bibcode:2005AnPhy.317..383F, doi:10.1016/j.aop.2004.11.008, S2CID 119632876 http://www.sciencedirect.com/science/article/pii/S0003491604002209 ↩

5. Francis & Kosowsky 2005, p. 386. - Francis; Kosowsky (2005), "The construction of spinors in geometric algebra", Annals of Physics, 317 (2): 317, 384–409, arXiv:math-ph/0403040, Bibcode:2005AnPhy.317..383F, doi:10.1016/j.aop.2004.11.008, S2CID 119632876 http://www.sciencedirect.com/science/article/pii/S0003491604002209 ↩

6. Hamilton 1853, p. 639. - Hamilton, William Rowan (1853), Lectures on Quaternions, Article 669, Hodges and Smith; [etc., etc.] https://catalog.hathitrust.org/Record/000470490 ↩

7. Hamilton 1853, p. 730. - Hamilton, William Rowan (1853), Lectures on Quaternions, Article 669, Hodges and Smith; [etc., etc.] https://catalog.hathitrust.org/Record/000470490 ↩

8. Hamilton 1866, p. 289. - Hamilton, William Rowan (1866), Hamilton, William Edwin (ed.), Elements of Quaternions (1st ed.), Longmans, Green & Co. https://books.google.com/books?id=fIRAAAAAIAAJ ↩

9. Dickson 1914, p. 13. - Dickson, Leonard (1914), Linear Algebras, §13 "Equivalence of the complex quaternion and matric algebras" https://babel.hathitrust.org/cgi/pt?id=uc1.b5008837;view=1up;seq=25 ↩

10. Hermann 1974, chapter 6.4 Complex Quaternions and Maxwell's Equations. - Hermann, Robert (1974), Spinors, Clifford and Cayley Algebras, Interdisciplinary Mathematics, vol. VII, Math Sci Press, pp. 250–265, ISBN 0-915692-06-6 ↩

11. Furey 2012. - Furey, C. (2012). "Unified Theory of Ideals". Phys. Rev. D. 86 (2): 025024. arXiv:1002.1497. Bibcode:2012PhRvD..86b5024F. doi:10.1103/PhysRevD.86.025024. S2CID 118458623. https://arxiv.org/abs/1002.1497 ↩