|
|
| Line 1: |
Line 1: |
| {{About|(orthogonal) Clifford algebra|symplectic Clifford algebra|Weyl algebra}}
| | Out of those 16 volcanoes, four of them are located in Europe. Providing world class facilities the Club La Santa complex is ideally situated for athletes who compete in outdoor sports but are unable to train during the winter months in Northern Europe. Just prior to quitting Fuengirola towards the end of the summer of '78 I'd been approached with an offer of concerts in the Canary Islands with the band, but turned them down as I was shortly due to begin as a drama student and so fulfill a dream come true. By isheldon : A how to tutorial about Lanzarote, Holiday Guide, Playa Blanca, Travel with step by step guide from isheldon. Its extinction was triggered by the introduction of firearms, which made hunting easier. There is always something to celebrate somewhere in Spain, from the very solemn to the purely pagan. These tunnels often get filled because of the tides, each creating a natural pool with its own beach. The shores of Lanzarote are very clear, mainly thanks to the volcanic rock that slopes into the Atlantic Ocean. The report states how hundreds of unaccompanied African migrant children are held in the government facilities located in the Canary Islands are at terrible risk of violence as well as ill treatment. Getting the school holidays right is key for any family booking a break abroad that everyone is going to enjoy. For more information about The Canary Islands please click on the following link www.lanzarote-hotels.co.uk - [http://Www.Chinjipet.com/?document_srl=747987 http://Www.Chinjipet.com] -.<br><br>The most popular seem to be at Heirro, The Bubble and El Muelle on the North coast and the quieter more remote waves are to be found at Punth Del Tigre and Playa de Pared in the south west. With car hire Tenerife feel bit of Playa de Las Americas, the old Tenerife. The majority wants to return to Morocco. Each song is honest and emotional but doesn't fall into that sappy category she expresses herself through an electric guitar and it's hot! In addition, we will always be guaranteed that the accommodation we have chosen is not far from the main tourist attractions. Tenerife has a number of the largest carnivals on earth. Tenerife is undoubtedly the biggest with the Canary Islands and also a hotspot for tourists, especially those that want to possess a wild time out. With the Tagus River wrapped around the city on four sides I understand why El Greco spent so many years painting and repainting this single landscape. The Canary Islands are one of the most popular destinations for both foreigners as well as the local Spanish people when it comes to vacations.<br><br>This resort is part of the Puerto Calma Group. The shores of Lanzarote are very clear, mainly thanks to the volcanic rock that slopes into the Atlantic Ocean. Tours are offered on the island to visit the different attractions and they include a breathtaking trip up and down Mount Teide. In the spring of 1978, I arrived in the famous Costa del Sol town of Fuengirola near Marbella, with the intention of helping to set up a sailing school with a young Englishman I knew only vaguely. They move on mainly because they weren't good enough to stay or certain circumstances prevented them from getting another chance. whenever something pleases you. Book the car hire Tenerife today itself for your Tenerife vacation and let every moment be livened up! It has a height of 1,488 meters (4,882 ft).<br><br>This grand and beautiful palace is situated in Granada which is famous among the tourists for the variety of historical places. The climate as an attraction Visitors say that the Islands enjoy an eternal spring. Lanzarote Beaches Lanzarote beaches are excellent. The Canary islands Date Palm's botanical name is Phoenix canariensis which makes it a close relative of the Date Palm (Phoenix dactylifera) we get dates from. Cyprus is blessed with in excess of 300 days of sunshine yearly. Legend has it though that Tenerife and other islands of the Canaries are the highest peaks of the legendary kingdom of Atlantis, and that they are all that's left after Atlantis sank. This island is a top notch tourist destination with the accommodation Tenerife facilities being of equal or better quality. They decimated. Everywhere was a repeated event, just like those in the Sahara: crushing weight, forced exhalation, struggle, and then oblivion.<br><br>The island's history and seafaring role are amply displayed". All seven Canary Islands offer great surfing conditions as the swell is consistent throughout the year. British tourists mainly travel to Gran Canaria for its wonderful climate all year round . The rain swept over the roof later that night and the rumbling of thunder rolled in the distance. The Herschel Telescope has capabilities for both optical and infrared imaging. The area, through a decree on August 9, 1974, has been very well-protected because of its ecological value and significance. Mount Vesuvius, Italy Mount Vesuvius was formed by the collision of African and the Eurasian tectonic plates. In Crete there are some famous nudists beaches, but check the laws, because they change often. The group of islands, which are part of Spain but located off Africa's north-west coast, have a great climate, with long, sunny days and soaring temperatures during the summer. |
| | |
| In [[mathematics]], '''Clifford algebras''' are a type of [[associative algebra]]. As [[algebra over a field|''K''-algebras]], they generalize the [[real number]]s, [[complex number]]s, [[quaternion]]s and several other [[hypercomplex number]] systems.<ref>W. K. Clifford, "Preliminary sketch of bi-quaternions, Proc. London Math. Soc. Vol. 4 (1873) pp. 381-395</ref><ref>W. K. Clifford, ''Mathematical Papers'', (ed. R. Tucker), London: Macmillan, 1882.</ref> The theory of Clifford algebras is intimately connected with the theory of [[quadratic form]]s and [[orthogonal group|orthogonal transformation]]s. Clifford algebras have important applications in a variety of fields including [[geometry]], [[theoretical physics]] and [[digital image processing]]. They are named after the English geometer [[William Kingdon Clifford]].
| |
| | |
| The most familiar Clifford algebra, or '''orthogonal Clifford algebra''', is also referred to as ''Riemannian Clifford algebra''.<ref>see for ex. Z. Oziewicz, Sz. Sitarczyk: ''Parallel treatment of Riemannian and symplectic Clifford algebras''. In: Artibano Micali, Roger Boudet, Jacques Helmstetter (eds.): ''Clifford Algebras and their Applications in Mathematical Physics'', Kluwer Academic Publishers, ISBN 0-7923-1623-1, 1992, [http://books.google.de/books?id=FhU9QpPIscoC&pg=PA83 p. 83]</ref> | |
| | |
| ==Introduction and basic properties==
| |
| A Clifford algebra is a [[unital algebra|unital]] [[associative algebra]] that contains and is generated by a [[vector space]] ''V'' over a [[Field (mathematics)|field]] ''K'', where ''V'' is equipped with a [[quadratic form]] ''Q''. The Clifford algebra ''C''ℓ(''V'', ''Q'') is the "freest" algebra generated by ''V'' subject to the condition<ref>Mathematicians who work with real Clifford algebras and prefer positive definite quadratic forms (especially those working in [[index theory]]) sometimes use a different [[sign convention|choice of sign]] in the fundamental Clifford identity. That is, they take {{nowrap|''v''<sup>2</sup> {{=}} −''Q''(''v'').}} One must replace ''Q'' with −''Q'' in going from one convention to the other.</ref>
| |
| :<math>v^2 = Q(v)1\ \text{ for all } v\in V,</math>
| |
| where the product on the left is that of the algebra, and the 1 is its [[multiplicative identity]].
| |
| | |
| The definition of a Clifford algebra endows it with more structure than a "bare" [[Algebra over a field|''K''-algebra]]: specifically it has a designated or privileged subspace that is [[isomorphism|isomorphic]] to ''V''. Such a subspace cannot in general be uniquely determined given only a ''K''-algebra isomorphic to the Clifford algebra.
| |
| | |
| If the [[characteristic (algebra)|characteristic]] of the ground [[field (mathematics)|field]] ''K'' is not 2, then one can rewrite this fundamental identity in the form
| |
| | |
| :<math>uv + vu = 2\lang u, v\rang1\ \text{ for all }u,v \in V,</math>
| |
| | |
| where
| |
| :<math> \langle u , v \rangle = \frac{1}{2} \left( Q(u+v) - Q(u) - Q(v) \right) </math>
| |
| is the [[symmetric matrix|symmetric]] [[bilinear form]] associated with ''Q'', via the [[polarization identity]]. The idea of being the "freest" or "most general" algebra subject to this identity can be formally expressed through the notion of a [[universal property]], as done [[#Universal property and construction|below]]. | |
| | |
| Quadratic forms and Clifford algebras in [[characteristic (algebra)|characteristic]] 2 form an exceptional case. In particular, if {{nowrap|char(''K'') {{=}} 2}} it is not true that a quadratic form determines a symmetric bilinear form, or that every quadratic form admits an orthogonal basis. Many of the statements in this article include the condition that the characteristic is not 2, and are false if this condition is removed.
| |
| | |
| ===As a quantization of the exterior algebra===
| |
| Clifford algebras are closely related to [[exterior algebra]]s. In fact, if {{nowrap|''Q'' {{=}} 0}} then the Clifford algebra ''C''ℓ(''V'', ''Q'') is just the exterior algebra Λ(''V''). For nonzero ''Q'' there exists a canonical ''linear'' isomorphism between Λ(''V'') and ''C''ℓ(''V'', ''Q'') whenever the ground field ''K'' does not have characteristic two. That is, they are [[naturally isomorphic]] as vector spaces, but with different multiplications (in the case of characteristic two, they are still isomorphic as vector spaces, just not naturally). Clifford multiplication together with the privileged subspace is strictly richer than the [[exterior product]] since it makes use of the extra information provided by ''Q''.
| |
| | |
| More precisely, Clifford algebras may be thought of as ''quantizations'' (cf. [[quantization (physics)]], [[Quantum group]]) of the exterior algebra, in the same way that the [[Weyl algebra]] is a quantization of the [[symmetric algebra]].
| |
| | |
| Weyl algebras and Clifford algebras admit a further structure of a [[*-algebra]], and can be unified as even and odd terms of a [[superalgebra]], as discussed in [[CCR and CAR algebras]].
| |
| | |
| == Universal property and construction ==
| |
| Let ''V'' be a [[vector space]] over a [[field (mathematics)|field]] ''K'', and let {{nobreak|''Q'': ''V'' → ''K''}} be a [[quadratic form]] on ''V''. In most cases of interest the field ''K'' is either '''R''', '''C''' or a [[finite field]].
| |
| | |
| A Clifford algebra ''C''ℓ(''V'', ''Q'') is a [[unital algebra|unital]] [[associative algebra]] over ''K'' together with a [[linear transformation|linear map]] {{nobreak|''i'' : ''V'' → ''C''ℓ(''V'', ''Q'')}} satisfying {{nobreak|''i''(''v'')<sup>2</sup> {{=}} ''Q''(''v'')1}} for all {{nobreak|''v'' ∈ ''V'',}} defined by the following [[universal property]]: given any associative algebra ''A'' over ''K'' and any linear map {{nobreak|''j'' : ''V'' → ''A''}} such that
| |
| | |
| :''j''(''v'')<sup>2</sup> = ''Q''(''v'')1<sub>''A''</sub> for all ''v'' ∈ ''V''
| |
| | |
| (where 1<sub>''A''</sub> denotes the multiplicative identity of ''A''), there is a unique [[algebra homomorphism]] {{nobreak|''f'' : ''C''ℓ(''V'', ''Q'') → ''A''}} such that the following diagram [[commutative diagram|commutes]] (i.e. such that {{nobreak|''f'' ∘ ''i'' {{=}} ''j''}}):
| |
| | |
| <div style="text-align: center;">[[Image:CliffordAlgebra-01.png]]</div>
| |
| | |
| Working with a symmetric [[bilinear form]] <·,·> instead of ''Q'' (in characteristic not 2), the requirement on ''j'' is
| |
| | |
| :<math> j(v)j(w) + j(w)j(v) = 2 \langle v, w \rangle 1_A \quad \mbox{ for all } v,w \in V \ . </math>
| |
| <!-- :''j''(''v'')''j''(''w'') + ''j''(''w'')''j''(''v'') = 2<''v'', ''w''>1<sub>''A''</sub> for all {{nobreak|''v'', ''w'' ∈ ''V''.}} -->
| |
| | |
| A Clifford algebra as described above always exists and can be constructed as follows: start with the most general algebra that contains ''V'', namely the [[tensor algebra]] ''T''(''V''), and then enforce the fundamental identity by taking a suitable [[quotient ring|quotient]]. In our case we want to take the [[ideal (ring theory)|two-sided ideal]] ''I<sub>Q</sub>'' in ''T''(''V'') generated by all elements of the form
| |
| | |
| :<math>v\otimes v - Q(v)1</math> for all <math>v\in V</math>
| |
| | |
| and define ''C''ℓ(''V'', ''Q'') as the quotient algebra
| |
| | |
| :''C''ℓ(''V'', ''Q'') = ''T''(''V'')/''I''<sub>''Q''.</sub>
| |
| | |
| The ring product inherited by this quotient is sometimes referred to as the '''Clifford product'''{{sfn|Lounesto|2001|loc=§1.8}} to differentiate it from the inner and outer products.
| |
| | |
| It is then straightforward to show that ''C''ℓ(''V'', ''Q'') contains ''V'' and satisfies the above universal property, so that ''C''ℓ is unique up to a unique isomorphism; thus one speaks of "the" Clifford algebra ''C''ℓ(''V'', ''Q''). It also follows from this construction that ''i'' is [[injective function|injective]]. One usually drops the ''i'' and considers ''V'' as a [[linear subspace]] of ''C''ℓ(''V'', ''Q'').
| |
| | |
| The universal characterization of the Clifford algebra shows that the construction of ''C''ℓ(''V'', ''Q'') is ''functorial'' in nature. Namely, ''C''ℓ can be considered as a [[functor]] from the [[category (mathematics)|category]] of vector spaces with quadratic forms (whose [[morphism]]s are linear maps preserving the quadratic form) to the category of associative algebras. The universal property guarantees that linear maps between vector spaces (preserving the quadratic form) extend uniquely to [[algebra homomorphism]]s between the associated Clifford algebras.
| |
| | |
| ==Basis and dimension==
| |
| If the [[dimension (linear algebra)|dimension]] of ''V'' is ''n'' and {''e''<sub>1</sub>, …, ''e''<sub>''n''</sub>} is a [[basis (linear algebra)|basis]] of ''V'', then the set
| |
| | |
| :<math>\{e_{i_1}e_{i_2}\cdots e_{i_k} \mid 1\le i_1 < i_2 < \cdots < i_k \le n\mbox{ and } 0\le k\le n\}</math>
| |
| | |
| is a basis for ''C''ℓ(''V'', ''Q''). The empty product ({{nobreak|1=''k'' = 0}}) is defined as the multiplicative [[identity element]]. For each value of ''k'' there are [[Binomial coefficient|''n'' choose ''k'']] basis elements, so the total dimension of the Clifford algebra is
| |
| | |
| :<math>\dim C\ell(V,Q) = \sum_{k=0}^n\begin{pmatrix}n\\ k\end{pmatrix} = 2^n.</math>
| |
| | |
| Since ''V'' comes equipped with a quadratic form, there is a set of privileged bases for ''V'': the [[orthogonal]] ones. An [[orthogonal basis]] is one such that
| |
| | |
| :<math>\langle e_i, e_j \rangle = 0 \qquad i\neq j. \,</math>
| |
| | |
| where ⟨·,·⟩ is the symmetric bilinear form associated to ''Q''. The fundamental Clifford identity implies that for an orthogonal basis
| |
| | |
| :<math>e_ie_j = -e_je_i \qquad i\neq j. \,</math>
| |
| | |
| This makes manipulation of orthogonal basis vectors quite simple. Given a product <math>e_{i_1}e_{i_2}\cdots e_{i_k}</math> of ''distinct'' orthogonal basis vectors of ''V'', one can put them into standard order while including an overall sign determined by the number of [[Transposition (mathematics)|pairwise swaps]] needed to do so (i.e. the [[Parity of a permutation|signature]] of the ordering [[permutation]]).
| |
| | |
| == Examples: real and complex Clifford algebras ==
| |
| The most important Clifford algebras are those over [[real number|real]] and [[complex number|complex]] vector spaces equipped with [[nondegenerate quadratic form]]s.
| |
| | |
| It turns out that every one of the algebras ''C''ℓ<sub>''p'',''q''</sub>('''R''') and ''C''ℓ<sub>''n''</sub>('''C''') is isomorphic to ''A'' or ''A''⊕''A'', where ''A'' is a [[Matrix ring|full matrix ring]] with entries from '''R''', '''C''', or '''H'''. For a complete classification of these algebras see [[classification of Clifford algebras]].
| |
| | |
| === Real numbers ===
| |
| {{main|Geometric algebra}}
| |
| The geometric interpretation of real Clifford algebras is known as [[geometric algebra]].
| |
| | |
| Every nondegenerate quadratic form on a finite-dimensional real vector space is equivalent to the standard diagonal form:
| |
| :<math>Q(v) = v_1^2 + \cdots + v_p^2 - v_{p+1}^2 - \cdots - v_{p+q}^2</math>
| |
| where {{nowrap|1=''n'' = ''p'' + ''q''}} is the dimension of the vector space. The pair of integers (''p'', ''q'') is called the [[metric signature|signature]] of the quadratic form. The real vector space with this quadratic form is often denoted '''R'''<sup>''p'', ''q''</sup>. The Clifford algebra on '''R'''<sup>''p'', ''q''</sup> is denoted ''C''ℓ<sub>''p'', ''q''</sub>('''R'''). The symbol ''C''ℓ<sub>''n''</sub>('''R''') means either ''C''ℓ<sub>''n'',0</sub>('''R''') or ''C''ℓ<sub>0,''n''</sub>('''R''') depending on whether the author prefers positive definite or negative definite spaces.
| |
| | |
| A standard [[orthonormal basis]] {''e''<sub>''i''</sub>} for '''R'''<sup>''p'',''q''</sup> consists of {{nowrap|1=''n'' = ''p'' + ''q''}} mutually orthogonal vectors, ''p'' of which have norm +1 and ''q'' of which have norm −1. The algebra ''C''ℓ<sub>''p'',''q''</sub>('''R''') will therefore have ''p'' vectors that square to +1 and ''q'' vectors that square to −1.
| |
| | |
| Note that ''C''ℓ<sub>0,0</sub>('''R''') is naturally isomorphic to '''R''' since there are no nonzero vectors. ''C''ℓ<sub>0,1</sub>('''R''') is a two-dimensional algebra generated by a single vector ''e''<sub>1</sub> that squares to −1, and therefore is isomorphic to '''C''', the field of [[complex number]]s. The algebra ''C''ℓ<sub>0,2</sub>('''R''') is a four-dimensional algebra spanned by {1, ''e''<sub>1</sub>, ''e''<sub>2</sub>, ''e''<sub>1</sub>''e''<sub>2</sub>}. The latter three elements square to −1 and all anticommute, and so the algebra is isomorphic to the [[quaternion]]s '''H'''. ''C''ℓ<sub>0,3</sub>('''R''') is an 8-dimensional algebra isomorphic to the [[Direct sum of modules#Direct sum of algebras|direct sum]] {{nowrap|'''H''' ⊕ '''H'''}} called [[split-biquaternion]]s.
| |
| | |
| === Complex numbers ===
| |
| One can also study Clifford algebras on complex vector spaces. Every nondegenerate quadratic form on a complex vector space is equivalent to the standard diagonal form
| |
| :<math>Q(z) = z_1^2 + z_2^2 + \cdots + z_n^2</math>
| |
| where {{nowrap|1=''n'' = dim ''V''}}, up to isomorphism so there is only one nondegenerate Clifford algebra for each dimension ''n''. We will denote the Clifford algebra on '''C'''<sup>''n''</sup> with the standard quadratic form by ''C''ℓ<sub>''n''</sub>('''C''').
| |
| | |
| The first few cases are not hard to compute. One finds that
| |
| :''C''ℓ<sub>0</sub>('''C''') ≅ '''C''', the [[complex number]]s
| |
| :''C''ℓ<sub>1</sub>('''C''') ≅ '''C''' ⊕ '''C''', the [[bicomplex number]]s
| |
| :''C''ℓ<sub>2</sub>('''C''') ≅ ''M''(2, '''C'''), the [[biquaternion]]s
| |
| where ''M''(''n'', '''C''') denotes the algebra of ''n''×''n'' matrices over '''C'''.
| |
| | |
| ==Examples: constructing quaternions and dual quaternions==
| |
| ===Quaternions===
| |
| In this section, Hamilton's [[quaternion]]s are constructed as the even sub algebra of the Clifford algebra ''C''ℓ<sub>0,3</sub>('''R''').
| |
| | |
| Let the vector space ''V'' be real three dimensional space '''R'''<sup>3</sup>, and the quadratic form Q be derived from the usual Euclidean metric. Then, for '''v''', '''w''' in '''R'''<sup>3</sup> we have the quadratic form, or dot product,
| |
| : <math>\mathbf{v}\cdot\mathbf{w}= v_1w_1 + v_2w_2 + v_3w_3.</math>
| |
| Now introduce the Clifford product of vectors '''v''' and '''w''' given by
| |
| :<math> \mathbf{v}\mathbf{w} + \mathbf{w}\mathbf{v} = -2 (\mathbf{v}\cdot \mathbf{w}).\!</math>
| |
| This formulation uses the negative sign so the correspondence with [[quaternion]]s is easily shown.
| |
| | |
| Denote a set of orthogonal unit vectors of '''R'''<sup>3</sup> as '''e'''<sub>1</sub>, '''e'''<sub>2</sub>, and '''e'''<sub>3</sub>, then the Clifford product yields the relations
| |
| :<math> \mathbf{e}_2 \mathbf{e}_3 = -\mathbf{e}_3 \mathbf{e}_2, \,\,\, \mathbf{e}_3 \mathbf{e}_1 = -\mathbf{e}_1 \mathbf{e}_3,\,\,\, \mathbf{e}_1 \mathbf{e}_2 = -\mathbf{e}_2 \mathbf{e}_1,\!</math>
| |
| and
| |
| :<math> \mathbf{e}_1 ^2 = \mathbf{e}_2^2 =\mathbf{e}_3^2 = -1. \!</math>
| |
| The general element of the Clifford algebra ''C''ℓ<sub>0,3</sub>('''R''') is given by
| |
| :<math> A = a_0 + a_1 \mathbf{e}_1 + a_2 \mathbf{e}_2 + a_3 \mathbf{e}_3 + a_4 \mathbf{e}_2 \mathbf{e}_3 + a_5 \mathbf{e}_3 \mathbf{e}_1 + a_6 \mathbf{e}_1 \mathbf{e}_2 + a_7 \mathbf{e}_1 \mathbf{e}_2 \mathbf{e}_3.\!</math>
| |
| | |
| The linear combination of the even grade elements of ''C''ℓ<sub>0,3</sub>('''R''') defines the even sub algebra ''C''ℓ<sup>0</sup><sub>0,3</sub>('''R''') with the general element
| |
| :<math> Q = q_0 + q_1 \mathbf{e}_2 \mathbf{e}_3 + q_2 \mathbf{e}_3 \mathbf{e}_1 + q_3 \mathbf{e}_1 \mathbf{e}_2. \!</math>
| |
| The basis elements can be identified with the quaternion basis elements ''i'', ''j'', ''k'' as
| |
| :<math> i= \mathbf{e}_2 \mathbf{e}_3, j= \mathbf{e}_3 \mathbf{e}_1, k = \mathbf{e}_1 \mathbf{e}_2,</math>
| |
| which shows that the even sub algebra ''C''ℓ<sup>0</sup><sub>0,3</sub>('''R''') is Hamilton's real [[quaternion]] algebra.
| |
| | |
| To see this, compute
| |
| :<math> i^2 = (\mathbf{e}_2 \mathbf{e}_3)^2 = \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_2 \mathbf{e}_3 = - \mathbf{e}_2 \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_3 = -1,\!</math>
| |
| and
| |
| :<math> ij = \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_3 \mathbf{e}_1 = -\mathbf{e}_2 \mathbf{e}_1 = \mathbf{e}_1 \mathbf{e}_2 = k.\!</math>
| |
| Finally,
| |
| :<math> ijk = \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_3 \mathbf{e}_1 \mathbf{e}_1 \mathbf{e}_2 = -1.\!</math>
| |
| | |
| ===Dual quaternions===
| |
| In this section, [[dual quaternion]]s are constructed as the even Clifford algebra of real four dimensional space with a degenerate quadratic form.<ref>[http://books.google.com/books?id=glOqQgAACAAJ&dq=inauthor:%22J.+M.+McCarthy%22&hl=en&ei=_QoMToDvMcfd0QGFh-mvDg&sa=X&oi=book_result&ct=book-thumbnail&resnum=3&ved=0CDsQ6wEwAg J. M. McCarthy, ''An Introduction to Theoretical Kinematics'', pp. 62–5, MIT Press 1990.]</ref><ref>[http://books.google.com/books?id=f8I4yGVi9ocC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false O. Bottema and B. Roth, ''Theoretical Kinematics'', North Holland Publ. Co., 1979]</ref>
| |
| | |
| Let the vector space V be real four dimensional space '''R'''<sup>4</sup>, and let the quadratic form ''Q'' be a degenerate form derived from the Euclidean metric on '''R'''<sup>3</sup>. For '''v''', '''w''' in '''R'''<sup>4</sup> introduce the degenerate bilinear form
| |
| : <math>d(\mathbf{v}, \mathbf{w})= v_1w_1 + v_2w_2 + v_3w_3.</math>
| |
| This degenerate scalar product projects distance measurements in '''R'''<sup>4</sup> onto the '''R'''<sup>3</sup> hyperplane.
| |
| | |
| The Clifford product of vectors '''v''' and '''w''' is given by
| |
| :<math> \mathbf{v}\mathbf{w} + \mathbf{w}\mathbf{v} = -2 \,d(\mathbf{v}, \mathbf{w}).\!</math>
| |
| Note the negative sign is introduced to simplify the correspondence with quaternions.
| |
| | |
| Denote a set of orthogonal unit vectors of '''R'''<sup>4</sup> as '''e'''<sub>1</sub>, '''e'''<sub>2</sub>, '''e'''<sub>3</sub> and '''e'''<sub>4</sub>, then the Clifford product yields the relations
| |
| :<math> \mathbf{e}_m \mathbf{e}_n = -\mathbf{e}_n \mathbf{e}_m, \,\,\, m \ne n,\!</math>
| |
| and
| |
| :<math> \mathbf{e}_1 ^2 = \mathbf{e}_2^2 =\mathbf{e}_3^2 = -1, \,\, \mathbf{e}_4^2 =0.\!</math>
| |
| | |
| The general element of the Clifford algebra ''C''ℓ('''R'''<sup>4</sup>,''d'') has 16 components. The linear combination of the even graded elements defines the even sub algebra ''C''ℓ<sup>0</sup>('''R'''<sup>4</sup>,''d'') with the general element
| |
| :<math> H = h_0 + h_1 \mathbf{e}_2 \mathbf{e}_3 + h_2 \mathbf{e}_3 \mathbf{e}_1 + h_3 \mathbf{e}_1 \mathbf{e}_2 + h_4 \mathbf{e}_4 \mathbf{e}_1 + h_5 \mathbf{e}_4 \mathbf{e}_2 + h_6 \mathbf{e}_4 \mathbf{e}_3 + h_7 \mathbf{e}_1 \mathbf{e}_2\mathbf{e}_3 \mathbf{e}_4. \!</math>
| |
| | |
| The basis elements can be identified with the quaternion basis elements ''i'', ''j'', ''k'' and the dual unit ''ε'' as
| |
| :<math> i=\mathbf{e}_2 \mathbf{e}_3, j=\mathbf{e}_3 \mathbf{e}_1, k = \mathbf{e}_1 \mathbf{e}_2, \,\, \varepsilon = \mathbf{e}_1 \mathbf{e}_2\mathbf{e}_3 \mathbf{e}_4. \!</math>
| |
| This provides the correspondence of ''C''ℓ<sup>0</sup><sub>0,3,1</sub>('''R''') with [[dual quaternion]] algebra.
| |
| | |
| To see this, compute
| |
| :<math> \varepsilon ^2 = (\mathbf{e}_1 \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_4)^2 = \mathbf{e}_1 \mathbf{e}_2\mathbf{e}_3 \mathbf{e}_4 \mathbf{e}_1 \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_4 = -\mathbf{e}_1 \mathbf{e}_2\mathbf{e}_3 (\mathbf{e}_4 \mathbf{e}_4 ) \mathbf{e}_1 \mathbf{e}_2\mathbf{e}_3 = 0,\!</math>
| |
| and
| |
| :<math> \varepsilon i = (\mathbf{e}_1 \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_4) \mathbf{e}_2 \mathbf{e}_3 = \mathbf{e}_1 \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_4 \mathbf{e}_2 \mathbf{e}_3 = \mathbf{e}_2\mathbf{e}_3 (\mathbf{e}_1 \mathbf{e}_2 \mathbf{e}_3 \mathbf{e}_4) = i\varepsilon.\!</math>
| |
| The exchanges of '''e'''<sub>1</sub> and '''e'''<sub>4</sub> alternate signs an even number of times, and show the dual unit ''ε'' commutes with the quaternion basis elements ''i'', ''j'', and ''k''.
| |
| | |
| ==Properties==
| |
| ===Relation to the exterior algebra===
| |
| Given a vector space ''V'' one can construct the [[exterior algebra]] Λ(''V''), whose definition is independent of any quadratic form on ''V''. It turns out that if ''K'' does not have characteristic 2 then there is a [[natural isomorphism]] between Λ(''V'') and ''C''ℓ(''V'', ''Q'') considered as vector spaces (and there exists an isomorphism in characteristic two, which may not be natural). This is an algebra isomorphism if and only if ''Q'' = 0. One can thus consider the Clifford algebra ''C''ℓ(''V'', ''Q'') as an enrichment (or more precisely, a quantization, cf. the Introduction) of the exterior algebra on ''V'' with a multiplication that depends on ''Q'' (one can still define the exterior product independent of ''Q'').
| |
| | |
| The easiest way to establish the isomorphism is to choose an ''orthogonal'' basis {''e<sub>i</sub>''} for ''V'' and extend it to a basis for ''C''ℓ(''V'', ''Q'') as described [[#Basis and dimension|above]]. The map {{nowrap|1=''C''ℓ(''V'', ''Q'') → Λ(''V'')}} is determined by
| |
| :<math>e_{i_1}e_{i_2}\cdots e_{i_k} \mapsto e_{i_1}\wedge e_{i_2}\wedge \cdots \wedge e_{i_k}.</math>
| |
| Note that this only works if the basis {''e''<sub>''i''</sub>} is orthogonal. One can show that this map is independent of the choice of orthogonal basis and so gives a natural isomorphism.
| |
| | |
| If the [[characteristic (algebra)|characteristic]] of ''K'' is 0, one can also establish the isomorphism by antisymmetrizing. Define functions {{nowrap|1=''f<sub>k</sub>'': ''V'' × … × ''V'' → ''C''ℓ(''V'', ''Q'')}} by
| |
| :<math>f_k(v_1, \cdots, v_k) = \frac{1}{k!}\sum_{\sigma\in S_k}{\rm sgn}(\sigma)\, v_{\sigma(1)}\cdots v_{\sigma(k)}</math>
| |
| where the sum is taken over the [[symmetric group]] on ''k'' elements. Since ''f<sub>k</sub>'' is [[alternating form|alternating]] it induces a unique linear map {{nowrap|1=Λ<sup>''k''</sup>(''V'') → ''C''ℓ(''V'', ''Q'')}}. The [[Direct sum of modules|direct sum]] of these maps gives a linear map between Λ(''V'') and ''C''ℓ(''V'', ''Q''). This map can be shown to be a linear isomorphism, and it is natural.
| |
| | |
| A more sophisticated way to view the relationship is to construct a [[filtration (abstract algebra)|filtration]] on ''C''ℓ(''V'', ''Q''). Recall that the [[tensor algebra]] ''T''(''V'') has a natural filtration: {{nowrap|1=''F''<sup>0</sup> ⊂ ''F''<sup>1</sup> ⊂ ''F''<sup>2</sup> ⊂ …}} where ''F<sup>k</sup>'' contains sums of tensors with [[tensor order|order]] {{nowrap|1=≤ ''k''}}. Projecting this down to the Clifford algebra gives a filtration on ''C''ℓ(''V'', ''Q''). The associated [[graded algebra]]
| |
| :<math>Gr_F C\ell(V,Q) = \bigoplus_k F^k/F^{k-1}</math>
| |
| is naturally isomorphic to the exterior algebra Λ(''V''). Since the associated graded algebra of a filtered algebra is always isomorphic to the filtered algebra as filtered vector spaces (by choosing complements of ''F<sup>k</sup>'' in ''F''<sup>''k''+1</sup> for all ''k''), this provides an isomorphism (although not a natural one) in any characteristic, even two.
| |
| | |
| ===Grading===
| |
| In the following, assume that the characteristic is not 2.<ref>Thus the [[group algebra]] '''K'''['''Z'''/2] is [[Semisimple algebra|semisimple]] and the Clifford algebra splits into eigenspaces of the main involution.</ref>
| |
| | |
| Clifford algebras are '''Z'''<sub>2</sub>-[[graded algebra]]s (also known as [[superalgebra]]s). Indeed, the linear map on ''V'' defined by ''v'' ↦ −''v'' ([[reflection through the origin]]) preserves the quadratic form ''Q'' and so by the universal property of Clifford algebras extends to an algebra [[automorphism]]
| |
| :α: ''C''ℓ(''V'', ''Q'') → ''C''ℓ(''V'', ''Q'').
| |
| Since α is an [[Involution (mathematics)|involution]] (i.e. it squares to the [[identity function|identity]]) one can decompose ''C''ℓ(''V'', ''Q'') into positive and negative eigenspaces of ''α''
| |
| :<math>C\ell(V,Q) = C\ell^0(V,Q) \oplus C\ell^1(V,Q)</math>
| |
| where ''C''ℓ<sup>''i''</sup>(''V'', ''Q'') = {''x'' ∈ ''C''ℓ(''V'', ''Q'') | ''α''(''x'') = (−1)<sup>''i''</sup>''x''}. Since ''α'' is an automorphism it follows that
| |
| :<math>C\ell^{\,i}(V,Q)C\ell^{\,j}(V,Q) = C\ell^{\,i+j}(V,Q)</math>
| |
| where the superscripts are read modulo 2. This gives ''C''ℓ(''V'', ''Q'') the structure of a '''Z'''<sub>2</sub>-[[graded algebra]]. The subspace ''C''ℓ<sup>0</sup>(''V'', ''Q'') forms a [[subalgebra]] of ''C''ℓ(''V'', ''Q''), called the ''even subalgebra''. The subspace ''C''ℓ<sup>1</sup>(''V'', ''Q'') is called the ''odd part'' of ''C''ℓ(''V'', ''Q'') (it is not a subalgebra). This '''Z'''<sub>2</sub>-grading plays an important role in the analysis and application of Clifford algebras. The automorphism ''α'' is called the ''main [[involution (mathematics)|involution]]'' or ''grade involution''. Elements that are pure in this '''Z'''<sub>2</sub>-grading are simply said to be even or odd.
| |
| | |
| ''Remark''. In characteristic not 2 the underlying vector space of ''C''ℓ(''V'', ''Q'') inherits an '''N'''-grading and a '''Z'''-grading from the canonical isomorphism with the underlying vector space of the exterior algebra Λ(''V'').<ref>The '''Z'''-grading is obtained from the '''N''' grading by appending copies of the zero subspace indexed with the negative integers.</ref> It is important to note, however, that this is a ''vector space grading only''. That is, Clifford multiplication does not respect the '''N'''-grading or '''Z'''-grading, only the '''Z'''<sub>2</sub>-grading: for instance if {{nowrap|1=''Q''(''v'') ≠ 0}}, then {{nowrap|1=''v'' ∈ ''C''ℓ<sup>1</sup>(''V'', ''Q'')}}, but {{nowrap|1=''v''<sup>2</sup> ∈ ''C''ℓ<sup>0</sup>(''V'', ''Q'')}}, not in {{nowrap|1=''C''ℓ<sup>2</sup>(''V'', ''Q'')}}. Happily, the gradings are related in the natural way: '''Z'''<sub>2</sub> ≅'''N'''/2'''N'''≅ '''Z'''/2'''Z'''. Further, the Clifford algebra is '''Z'''-[[filtered algebra|filtered]]: {{nowrap|1=''C''ℓ<sup>≤''i''</sup>(''V'', ''Q'') ⋅ ''C''ℓ<sup>≤''j''</sup>(''V'', ''Q'') ⊂ ''C''ℓ<sup>≤''i''+''j''</sup>(''V'', ''Q'')}}. The ''degree'' of a Clifford number usually refers to the degree in the '''N'''-grading.
| |
| | |
| The even subalgebra ''C''ℓ<sup>0</sup>(''V'', ''Q'') of a Clifford algebra is itself isomorphic to a Clifford algebra.<ref>Technically, it does not have the full structure of a Clifford algebra without a designated vector subspace.</ref><ref>We are still assuming that the characteristic is not 2.</ref> If ''V'' is the [[orthogonal direct sum]] of a vector ''a'' of norm ''Q''(''a'') and a subspace ''U'', then ''C''ℓ<sup>0</sup>(''V'', ''Q'') is isomorphic to ''C''ℓ(''U'', −''Q''(''a'')''Q''), where −''Q''(''a'')''Q'' is the form ''Q'' restricted to ''U'' and multiplied by −''Q''(''a''). In particular over the reals this implies that
| |
| :<math>C\ell_{p,q}^0(\mathbf{R}) \cong C\ell_{p,q-1}(\mathbf{R})</math> for ''q'' > 0, and
| |
| :<math>C\ell_{p,q}^0(\mathbf{R}) \cong C\ell_{q,p-1}(\mathbf{R})</math> for ''p'' > 0.
| |
| In the negative-definite case this gives an inclusion ''C''ℓ<sub>0,''n''−1</sub>('''R''') ⊂ ''C''ℓ<sub>0,''n''</sub>('''R''') which extends the sequence
| |
| :'''R''' ⊂ '''C''' ⊂ '''H''' ⊂ '''H'''⊕'''H''' ⊂ …;
| |
| Likewise, in the complex case, one can show that the even subalgebra of ''C''ℓ<sub>''n''</sub>('''C''') is isomorphic to ''C''ℓ<sub>''n''−1</sub>('''C''').
| |
| | |
| ===Antiautomorphisms===
| |
| In addition to the automorphism ''α'', there are two [[antiautomorphism]]s which play an important role in the analysis of Clifford algebras. Recall that the [[tensor algebra]] ''T''(''V'') comes with an antiautomorphism that reverses the order in all products:
| |
| :<math>v_1\otimes v_2\otimes \cdots \otimes v_k \mapsto v_k\otimes \cdots \otimes v_2\otimes v_1.</math>
| |
| Since the ideal ''I''<sub>''Q''</sub> is invariant under this reversal, this operation descends to an antiautomorphism of ''C''ℓ(''V'', ''Q'') called the ''[[Transposition (mathematics)|transpose]]'' or ''reversal'' operation, denoted by ''x<sup>t</sup>''. The transpose is an antiautomorphism: (''xy'')<sup>''t''</sup> = ''y<sup>t</sup> x<sup>t</sup>''. The transpose operation makes no use of the '''Z'''<sub>2</sub>-grading so we define a second antiautomorphism by composing ''α'' and the transpose. We call this operation ''Clifford conjugation'' denoted <math>\bar x</math>
| |
| :<math>\bar x = \alpha(x^t) = \alpha(x)^t.</math>
| |
| Of the two antiautomorphisms, the transpose is the more fundamental.<ref>The opposite is true when using the alternate (−) sign convention for Clifford algebras: it is the conjugate which is more important. In general, the meanings of conjugation and transpose are interchanged when passing from one sign convention to the other. For example, in the convention used here the inverse of a vector is given by {{math|''v''<sup>−1</sup> {{=}} ''v''<sup>''t''</sup> / ''Q''(''v'')}} while in the (−) convention it is given by {{math|''v''<sup>−1</sup> {{=}} {{overline|''v''}} / ''Q''(''v'')}}.</ref>
| |
| | |
| Note that all of these operations are [[involution (mathematics)|involutions]]. One can show that they act as ±1 on elements which are pure in the '''Z'''-grading. In fact, all three operations depend only on the degree modulo 4. That is, if ''x'' is pure with degree ''k'' then
| |
| :<math>\alpha(x) = \pm x \qquad x^t = \pm x \qquad \bar x = \pm x</math>
| |
| where the signs are given by the following table:
| |
| {| border=1 style="margin-left: 2em; text-align: center;" cellpadding=4
| |
| ! ''k'' mod 4 || 0 || 1 || 2 || 3 ||
| |
| |-
| |
| | <math>\alpha(x)\,</math> || + || − || + || −
| |
| | (−1)<sup>''k''</sup>
| |
| |-
| |
| | <math>x^t\,</math> || + || + || − || −
| |
| | (−1)<sup>''k''(''k''−1)/2</sup>
| |
| |-
| |
| | <math>\bar x</math> || + || − || − || +
| |
| | (−1)<sup>''k''(''k''+1)/2</sup>
| |
| |}
| |
| | |
| ===Clifford scalar product===
| |
| When the characteristic is not 2, the quadratic form ''Q'' on ''V'' can be extended to a quadratic form on all of ''C''ℓ(''V'', ''Q'') (which we also denoted by ''Q''). A basis independent definition of one such extension is
| |
| :<math>Q(x) = \lang x^t x\rang</math>
| |
| where ⟨''a''⟩ denotes the scalar part of ''a'' (the grade 0 part in the '''Z'''-grading). One can show that
| |
| :<math>Q(v_1v_2\cdots v_k) = Q(v_1)Q(v_2)\cdots Q(v_k)</math>
| |
| where the ''v<sub>i</sub>'' are elements of ''V'' – this identity is ''not'' true for arbitrary elements of ''C''ℓ(''V'', ''Q'').
| |
| | |
| The associated symmetric bilinear form on ''C''ℓ(''V'', ''Q'') is given by
| |
| :<math>\lang x, y\rang = \lang x^t y\rang.</math>
| |
| One can check that this reduces to the original bilinear form when restricted to ''V''. The bilinear form on all of ''C''ℓ(''V'', ''Q'') is [[nondegenerate form|nondegenerate]] if and only if it is nondegenerate on ''V''.
| |
| | |
| It is not hard to verify that the transpose is the [[adjoint of an operator|adjoint]] of left/right Clifford multiplication with respect to this inner product. That is,
| |
| :<math>\lang ax, y\rang = \lang x, a^t y\rang,</math>
| |
| and
| |
| :<math>\lang xa, y\rang = \lang x, y a^t\rang.</math>
| |
| | |
| ==Structure of Clifford algebras==
| |
| In this section we assume that the vector space ''V'' is finite dimensional and that the bilinear form of ''Q'' is non-singular. A [[central simple algebra]] over ''K'' is a matrix algebra over a (finite dimensional) division algebra with center ''K''. For example, the central simple algebras over the reals are matrix algebras over either the reals or the quaternions.
| |
| | |
| *If ''V'' has even dimension then ''C''ℓ(''V'', ''Q'') is a central simple algebra over ''K''.
| |
| *If ''V'' has even dimension then ''C''ℓ<sup>0</sup>(''V'', ''Q'') is a central simple algebra over a quadratic extension of ''K'' or a sum of two isomorphic central simple algebras over ''K''.
| |
| *If ''V'' has odd dimension then ''C''ℓ(''V'', ''Q'') is a central simple algebra over a quadratic extension of ''K'' or a sum of two isomorphic central simple algebras over ''K''.
| |
| *If ''V'' has odd dimension then ''C''ℓ<sup>0</sup>(''V'', ''Q'') is a central simple algebra over ''K''.
| |
| | |
| The structure of Clifford algebras can be worked out explicitly using the following result. Suppose that ''U'' has even dimension and a non-singular bilinear form with [[discriminant]] ''d'', and suppose that ''V'' is another vector space with a quadratic form. The Clifford algebra of ''U''+''V'' is isomorphic to the tensor product of the Clifford algebras of ''U'' and (−1)<sup>dim(''U'')/2</sup>''dV'', which is the space ''V'' with its quadratic form multiplied by (−1)<sup>dim(''U'')/2</sup>''d''. Over the reals, this implies in particular that
| |
| :<math> C\ell_{p+2,q}(\mathbf{R}) = M_2(\mathbf{R})\otimes C\ell_{q,p}(\mathbf{R}) </math>
| |
| :<math> C\ell_{p+1,q+1}(\mathbf{R}) = M_2(\mathbf{R})\otimes C\ell_{p,q}(\mathbf{R}) </math>
| |
| :<math> C\ell_{p,q+2}(\mathbf{R}) = \mathbf{H}\otimes C\ell_{q,p}(\mathbf{R}). </math>
| |
| These formulas can be used to find the structure of all real Clifford algebras and all complex Clifford algebras; see the [[classification of Clifford algebras]].
| |
| | |
| Notably, the [[Morita equivalence]] class of a Clifford algebra (its representation theory: the equivalence class of the category of modules over it) depends only on the signature {{nowrap|1=(''p'' − ''q'') mod 8}}. This is an algebraic form of [[Bott periodicity]].
| |
| | |
| ==Clifford group==
| |
| In this section we assume that ''V'' is finite dimensional and the quadratic form ''Q'' is [[nondegenerate form|nondegenerate]].
| |
| | |
| The invertible elements of the Clifford algebra act on it by twisted conjugation: conjugation by ''x'' maps ''y'' ↦ ''xy'' α(''x'')<sup>−1</sup>.
| |
| | |
| The Clifford group Γ is defined to be the set of invertible elements ''x'' that ''stabilize vectors'', meaning that for all ''v'' in ''V'' we have:
| |
| :<math>x v \alpha(x)^{-1}\in V</math>
| |
| | |
| This formula also defines an action of the Clifford group on the vector space ''V'' that preserves the norm ''Q'', and so gives a homomorphism from the Clifford group to the orthogonal group. The Clifford group contains all elements ''r'' of ''V'' of nonzero norm, and these act on ''V'' by the corresponding reflections that take ''v'' to {{nobreak|''v''−''2''⟨''v'',''r''⟩''r''/''Q''(''r'')}} (In characteristic 2 these are called orthogonal transvections rather than reflections.)
| |
| | |
| The Clifford group Γ is the disjoint union of two subsets Γ<sup>0</sup> and Γ<sup>1</sup>, where Γ<sup>''i''</sup> is the subset of elements of degree ''i''. The subset Γ<sup>0</sup> is a subgroup of [[Index of a subgroup|index]] 2 in Γ.
| |
| | |
| If ''V'' is a finite dimensional real vector space with positive definite (or negative definite) quadratic form then the Clifford group maps onto the orthogonal group of ''V'' with respect to the form (by the [[Cartan–Dieudonné theorem]]) and the kernel consists of the nonzero elements of the field ''K''. This leads to exact sequences
| |
| :<math> 1 \rightarrow K^* \rightarrow \Gamma \rightarrow \mbox{O}_V(K) \rightarrow 1,\,</math>
| |
| :<math> 1 \rightarrow K^* \rightarrow \Gamma^0 \rightarrow \mbox{SO}_V(K) \rightarrow 1.\,</math>
| |
| | |
| Over other fields or with indefinite forms, the map is not in general onto, and the failure is captured by the spinor norm.
| |
| | |
| ===Spinor norm===
| |
| {{details|Spinor_norm#Galois_cohomology_and_orthogonal_groups}}
| |
| | |
| In arbitrary characteristic, the [[Orthogonal group#The spinor norm|spinor norm]] ''Q'' is defined on the Clifford group by
| |
| :<math>Q(x) = x^tx.\,</math>
| |
| <!-- Note that (−1)^D(x) is the same as \alpha(x), so this expression is the same
| |
| as \alpha(x)^t x (−1)^D(x). -->
| |
| It is a homomorphism from the Clifford group to the group ''K*'' of non-zero elements of ''K''. It coincides with the quadratic form ''Q'' of ''V'' when ''V'' is identified with a subspace of the Clifford algebra. Several authors define the spinor norm slightly differently, so that it differs from the one here by a factor of −1, 2, or −2 on Γ<sup>1</sup>. The difference is not very important in characteristic other than 2.
| |
| | |
| The nonzero elements of ''K'' have spinor norm in the group ''K*''<sup>2</sup> of squares of nonzero elements of the field ''K''. So when ''V'' is finite dimensional and non-singular we get an induced map from the orthogonal group of ''V'' to the group ''K*''/''K*''<sup>2</sup>, also called the spinor norm. The spinor norm of the reflection of a vector ''r'' has image ''Q''(''r'') in ''K*''/''K*''<sup>2</sup>, and this property uniquely defines it on the orthogonal group. This gives exact sequences:
| |
| | |
| :<math> 1 \to \{\pm 1\} \to \mbox{Pin}_V(K) \to \mbox{O}_V(K) \to K^*/K^{*2},\,</math>
| |
| :<math> 1 \to \{\pm 1\} \to \mbox{Spin}_V(K) \to \mbox{SO}_V(K) \to K^*/K^{*2}.\,</math>
| |
| | |
| Note that in characteristic 2 the group {±1} has just one element.
| |
| | |
| From the point of view of [[Galois cohomology]] of [[algebraic group]]s, the spinor norm is a [[connecting homomorphism]] on cohomology. Writing μ<sub>2</sub> for the [[Group scheme of roots of unity|algebraic group of square roots of 1]] (over a field of characteristic not 2 it is roughly the same as a two-element group with trivial Galois action), the short exact sequence
| |
| :<math> 1 \to \mu_2 \rightarrow \mbox{Pin}_V \rightarrow \mbox{O}_V \rightarrow 1\,</math>
| |
| yields a long exact sequence on cohomology, which begins
| |
| :<math> 1 \to H^0(\mu_2;K) \to H^0(\mbox{Pin}_V;K) \to H^0(\mbox{O}_V;K) \to H^1(\mu_2;K).\,</math>
| |
| The 0th Galois cohomology group of an algebraic group with coefficients in ''K'' is just the group of ''K''-valued points: ''H''<sup>0</sup>(''G''; ''K'') = ''G''(''K''), and ''H''<sup>1</sup>(μ<sub>2</sub>; ''K'') ≅ ''K*''/''K*''<sup>2</sup>, which recovers the previous sequence
| |
| :<math> 1 \to \{\pm 1\} \to \mbox{Pin}_V(K) \to \mbox{O}_V(K) \to K^*/K^{*2},\,</math>
| |
| where the spinor norm is the connecting homomorphism ''H''<sup>0</sup>(O<sub>''V''</sub>; ''K'') → ''H''<sup>1</sup>(μ<sub>2</sub>; ''K'').
| |
| | |
| ==Spin and Pin groups==
| |
| {{details3|[[Spin group]], [[Pin group]] and [[spinor]]}}
| |
| | |
| In this section we assume that ''V'' is finite dimensional and its bilinear form is non-singular. (If ''K'' has characteristic 2 this implies that the dimension of ''V'' is even.)
| |
| | |
| The [[Pin group]] Pin<sub>''V''</sub>(''K'') is the subgroup of the Clifford group Γ of elements of spinor norm 1, and similarly the [[Spin group]] Spin<sub>''V''</sub>(''K'') is the subgroup of elements of [[Orthogonal group#The Dickson invariant|Dickson invariant]] 0 in Pin<sub>''V''</sub>(''K''). When the characteristic is not 2, these are the elements of determinant 1. The Spin group usually has index 2 in the Pin group.
| |
| | |
| Recall from the previous section that there is a homomorphism from the Clifford group onto the orthogonal group. We define the [[special orthogonal group]] to be the image of Γ<sup>0</sup>. If ''K'' does not have characteristic 2 this is just the group of elements of the orthogonal group of determinant 1. If ''K'' does have characteristic 2, then all elements of the orthogonal group have determinant 1, and the special orthogonal group is the set of elements of Dickson invariant 0.
| |
| | |
| There is a homomorphism from the Pin group to the orthogonal group. The image consists of the elements of spinor norm 1 ∈ ''K*''/''K*''<sup>2</sup>. The kernel consists of the elements +1 and −1, and has order 2 unless ''K'' has characteristic 2. Similarly there is a homomorphism from the Spin group to the special orthogonal group of ''V''.
| |
| | |
| In the common case when ''V'' is a positive or negative definite space over the reals, the spin group maps onto the special orthogonal group, and is simply connected when ''V'' has dimension at least 3. Further the kernel of this homomorphism consists of 1 and −1. So in this case the spin group, Spin(''n''), is a double cover of SO(''n''). Please note, however, that the simple connectedness of the spin group is not true in general: if ''V'' is '''R'''<sup>''p'',''q''</sup> for ''p'' and ''q'' both at least 2 then the spin group is not simply connected. In this case the algebraic group Spin<sub>''p'',''q''</sub> is simply connected as an algebraic group, even though its group of real valued points Spin<sub>''p'',''q''</sub>('''R''') is not simply connected. This is a rather subtle point, which completely confused the authors of at least one standard book about spin groups.
| |
| | |
| ==Spinors==<!-- This section is linked from [[Spinor]] -->
| |
| Clifford algebras ''C''ℓ<sub>''p'',''q''</sub>('''C'''), with ''p''+''q''=2''n'' even, are matrix algebras which have a complex representation of dimension 2<sup>''n''</sup>. By restricting to the group Pin<sub>''p'',''q''</sub>('''R''') we get a complex representation of the Pin group of the same dimension, called the [[spin representation]]. If we restrict this to the spin group Spin<sub>''p'',''q''</sub>('''R''') then it splits as the sum of two ''half spin representations'' (or ''Weyl representations'') of dimension 2<sup>''n''−1</sup>.
| |
| | |
| If ''p''+''q''=2''n''+1 is odd then the Clifford algebra ''C''ℓ<sub>''p'',''q''</sub>('''C''') is a sum of two matrix algebras, each of which has a representation of dimension 2<sup>''n''</sup>, and these are also both representations of the Pin group Pin<sub>''p'',''q''</sub>('''R'''). On restriction to the spin group Spin<sub>''p'',''q''</sub>('''R''') these become isomorphic, so the spin group has a complex spinor representation of dimension 2<sup>''n''</sup>.
| |
| | |
| More generally, spinor groups and pin groups over any field have similar representations whose exact structure depends on the [[classification of Clifford algebras|structure of the corresponding Clifford algebras]]: whenever a Clifford algebra has a factor that is a matrix algebra over some division algebra, we get a corresponding representation of the pin and spin groups over that division algebra.
| |
| For examples over the reals see the article on [[spinor]]s.
| |
| | |
| ===Real spinors===
| |
| {{details|spinor}}
| |
| To describe the real spin representations, one must know how the spin group sits inside its Clifford algebra. The [[Pin group]], Pin<sub>''p'',''q''</sub> is the set of invertible elements in ''C''ℓ<sub>''p'', ''q''</sub> which can be written as a product of unit vectors:
| |
| :<math>{\mbox{Pin}}_{p,q}=\{v_1v_2\dots v_r |\,\, \forall i\, \|v_i\|=\pm 1\}.</math>
| |
| Comparing with the above concrete realizations of the Clifford algebras, the Pin group corresponds to the products of arbitrarily many reflections: it is a cover of the full orthogonal group O(''p'', ''q''). The [[Spin group]] consists of those elements of Pin<sub>''p'', ''q''</sub> which are products of an even number of unit vectors. Thus by the [[Cartan-Dieudonné theorem]] Spin is a cover of the group of proper rotations SO(''p'',''q'').
| |
| | |
| Let ''α'' : ''C''ℓ → ''C''ℓ be the automorphism which is given by the mapping ''v'' ↦ −''v'' acting on pure vectors. Then in particular, Spin<sub>''p'',''q''</sub> is the subgroup of Pin<sub>''p'', ''q''</sub> whose elements are fixed by α. Let
| |
| :<math>C\ell_{p,q}^0 = \{ x\in C\ell_{p,q} |\, \alpha(x)=x\}.</math>
| |
| (These are precisely the elements of even degree in ''C''ℓ<sub>''p'', ''q''</sub>.) Then the spin group lies within ''C''ℓ<sup>0</sup><sub>''p'', ''q''</sub>.
| |
| | |
| The irreducible representations of ''C''ℓ<sub>''p'', ''q''</sub> restrict to give representations of the pin group. Conversely, since the pin group is generated by unit vectors, all of its irreducible representation are induced in this manner. Thus the two representations coincide. For the same reasons, the irreducible representations of the spin coincide with the irreducible representations of ''C''ℓ<sup>0</sup><sub>''p'', ''q''</sub>
| |
| | |
| To classify the pin representations, one need only appeal to the [[classification of Clifford algebras]]. To find the spin representations (which are representations of the even subalgebra), one can first make use of either of the isomorphisms (see above)
| |
| :''C''ℓ<sup>0</sup><sub>''p'',''q''</sub> ≈ ''C''ℓ<sub>''p'',''q''−1</sub>, for ''q'' > 0
| |
| :''C''ℓ<sup>0</sup><sub>''p'',''q''</sub> ≈ ''C''ℓ<sub>''q'',''p''−1</sub>, for ''p'' > 0
| |
| and realize a spin representation in signature (''p'',''q'') as a pin representation in either signature (''p'',''q''−1) or (''q'',''p''−1).
| |
| | |
| ==Applications==
| |
| ===Differential geometry===
| |
| One of the principal applications of the exterior algebra is in [[differential geometry]] where it is used to define the [[fiber bundle|bundle]] of [[differential form]]s on a [[smooth manifold]]. In the case of a ([[pseudo-Riemannian manifold|pseudo]]-)[[Riemannian manifold]], the [[tangent space]]s come equipped with a natural quadratic form induced by the [[metric tensor|metric]]. Thus, one can define a [[Clifford bundle]] in analogy with the [[exterior bundle]]. This has a number of important applications in [[Riemannian geometry]]. Perhaps more importantly is the link to a [[spin manifold]], its associated [[spinor bundle]] and spin<sup>c</sup> manifolds.
| |
| | |
| ===Physics===
| |
| Clifford algebras have numerous important applications in physics. Physicists usually consider a Clifford algebra to be an algebra spanned by matrices γ<sub>0</sub>,…,γ<sub>3</sub> called [[Dirac matrices]] which have the property that
| |
| | |
| :<math>\gamma_i\gamma_j + \gamma_j\gamma_i = 2\eta_{ij}\,</math>
| |
| | |
| where η is the matrix of a quadratic form of signature (1,3). These are exactly the defining relations for the Clifford algebra ''C''ℓ<sub>1,3</sub>('''C''') (up to an unimportant factor of 2), which by the [[classification of Clifford algebras]] is isomorphic to the algebra of 4 by 4 complex matrices.
| |
| | |
| The Dirac matrices were first written down by [[Paul Dirac]] when he was trying to write a relativistic first-order wave equation for the [[electron]], and give an explicit isomorphism from the Clifford algebra to the algebra of complex matrices. The result was used to define the [[Dirac equation]] and introduce the [[Dirac operator]]. The entire Clifford algebra shows up in [[quantum field theory]] in the form of [[Dirac field bilinear]]s.
| |
| | |
| The use of Clifford algebras to describe quantum theory has been advanced among others by [[Mario Schönberg]],<ref>See the references to Schönberg's papers of 1956 and 1957 as described in section "The Grassmann–Schönberg algebra <math>G_n</math>" of:A. O. Bolivar,
| |
| Classical limit of fermions in phase space, J. Math. Phys. 42, 4020 (2001) {{DOI|10.1063/1.1386411}}</ref> by [[David Hestenes]] in terms of [[geometric calculus]], by [[David Bohm]] and [[Basil Hiley]] and co-workers in form of a [[Basil Hiley#Hierarchy of Clifford algebras|hierarchy of Clifford algebras]], and by Elio Conte et al.<ref>E. Conte: The solution of EPR paradox in quantum mechanics, in: Fundamental Problems of Natural Sciences and Engineering, pp. 271–204, Saint Petersburg, 2002</ref><ref>Elio Conte: On some considerations of mathematical physics: May we identify Clifford algebra as a common algebraic structure for classical diffusion and Schrödinger equations? Adv. Studies Theor. Phys., vol. 6, no. 26 (2012), pp. 1289–1307</ref>
| |
| | |
| ===Computer Vision===
| |
| Recently, Clifford algebras have been applied in the problem of action recognition and classification in [[computer vision]]. Rodriguez et al.<ref name=Rodriguez2008>{{cite conference
| |
| | author = Rodriguez, Mikel
| |
| | coauthors = Shah, M
| |
| | year = 2008
| |
| | title = Action MACH: A Spatio-Temporal Maximum Average Correlation Height Filter for Action Classification
| |
| | booktitle = Computer Vision and Pattern Recognition (CVPR)
| |
| }}</ref> propose a Clifford embedding to generalize traditional MACH filters to video (3D spatiotemporal volume), and vector-valued data such as [[optical flow]]. Vector-valued data is analyzed using the [[Clifford Fourier transform]]. Based on these vectors action filters are synthesized in the Clifford Fourier domain and recognition of actions is performed using Clifford Correlation. The authors demonstrate the effectiveness of the Clifford embedding by recognizing actions typically performed in classic feature films and sports broadcast television.
| |
| | |
| ==See also==
| |
| {{Div col}}
| |
| *[[Algebra of physical space]], APS
| |
| *[[Classification of Clifford algebras]]
| |
| *[[Clifford module]]
| |
| *[[Gamma matrices]]
| |
| *[[Exterior algebra]]
| |
| *[[Generalized Clifford algebra]]
| |
| *[[Geometric algebra]]
| |
| *[[Spin group]]
| |
| *[[Spinor]]
| |
| *[[Paravector]]
| |
| *[[Cayley–Dickson construction]]
| |
| *[[spinor bundle]]
| |
| *[[Dirac operator]]
| |
| *[[Clifford analysis]]
| |
| *[[spin structure]]
| |
| *[[quaternion]]
| |
| *[[octonion]]
| |
| *[[complex spin structure]]
| |
| *[[hypercomplex number]]
| |
| {{Div col end}}
| |
| | |
| ==Notes==
| |
| {{Reflist|30em}}
| |
| | |
| ==References==
| |
| * {{Citation | last1=Bourbaki | first1=Nicolas | author1-link= Nicolas Bourbaki | title=Algebra | publisher=[[Springer-Verlag]] | location=Berlin, New York | isbn=978-3-540-19373-9 | year=1988}}, section IX.9.
| |
| *Carnahan, S. ''Borcherds Seminar Notes, Uncut.'' Week 5, "Spinors and Clifford Algebras".
| |
| *{{citation |zbl=1235.15025 | last=Garling | first=D. J. H. | title=Clifford algebras. An introduction | series=London Mathematical Society Student Texts | volume=78 | location=[[Cambridge]] | publisher=[[Cambridge University Press]] | year=2011 | isbn=978-1-107-09638-7 }}
| |
| * {{citation | title=Introduction to Quadratic Forms over Fields | volume=67 | series=Graduate Studies in Mathematics | first=Tsit-Yuen | last=Lam | authorlink=Tsit Yuen Lam | publisher=[[American Mathematical Society]] | year=2005 | isbn=0-8218-1095-2 | zbl=1068.11023 | mr = 2104929 }}
| |
| * {{Citation | last1=Lawson | first1=H. Blaine | last2=Michelsohn | first2=Marie-Louise|author2-link=Marie-Louise Michelsohn | title=Spin Geometry | publisher=[[Princeton University Press]] | location=Princeton, NJ | isbn=978-0-691-08542-5 | year=1989}}. An advanced textbook on Clifford algebras and their applications to differential geometry.
| |
| * {{Citation | last1=Lounesto | first1=Pertti | title=Clifford algebras and spinors | publisher=[[Cambridge University Press]] | location=Cambridge | isbn=978-0-521-00551-7 | year=2001}}
| |
| * {{Citation | last1=Porteous | first1=Ian R. | authorlink=Ian R. Porteous | title=Clifford algebras and the classical groups | publisher=[[Cambridge University Press]] | location=[[Cambridge]] | isbn=978-0-521-55177-9 | year=1995}}
| |
| * {{citation | first=R. | last=Jagannathan | arxiv=1005.4300 | title=On generalized Clifford algebras and their physical applications }}
| |
| * Sylvester, J. J., (1882), ''Johns Hopkins University Circulars'' '''I''': 241-242; ibid '''II''' (1883) 46; ibid '''III''' (1884) 7-9. Summarized in ''The Collected Mathematics Papers of James Joseph Sylvester'' (Cambridge University Press, 1909) v '''III''' .[http://quod.lib.umich.edu/u/umhistmath/aas8085.0003.001/664?rgn=full+text;view=pdf;q1=nonions online] and [http://quod.lib.umich.edu/u/umhistmath/AAS8085.0004.001/165?cite1=Sylvester;cite1restrict=author;rgn=full+text;view=pdf further].
| |
| | |
| ==Further reading==
| |
| * {{citation | last=Knus | first=Max-Albert | title=Quadratic and Hermitian forms over rings | series=Grundlehren der Mathematischen Wissenschaften | volume=294 | location=Berlin etc. | publisher=[[Springer-Verlag]] | year=1991 | isbn=3-540-52117-8 | zbl=0756.11008 }}
| |
| | |
| ==External links==
| |
| *{{springer|title=Clifford algebra|id=p/c022460}}
| |
| *[http://planetmath.org/encyclopedia/CliffordAlgebra2.html Planetmath entry on Clifford algebras]
| |
| *[http://members.fortunecity.com/jonhays/clifhistory.htm A history of Clifford algebras] (unverified)
| |
| *[http://www.math.ucr.edu/home/baez/octonions/node6.html John Baez on Clifford algebras]
| |
| | |
| {{DEFAULTSORT:Clifford Algebra}}
| |
| [[Category:Clifford algebras|*]]
| |
| [[Category:Ring theory]]
| |
| [[Category:Quadratic forms]]
| |