- Exterior algebra
In

mathematics , the**exterior product**or**wedge product**of vectors is an algebraic construction generalizing certain features of thecross product to higher dimensions. Like the cross product, and the scalartriple product , the exterior product of vectors is used inEuclidean geometry to studyarea s,volume s, and their higher-dimensional analogs. Inlinear algebra , the exterior product provides an abstract algebraic basis-independent manner for describing thedeterminant and the minors of alinear transformation , and is fundamentally related to ideas of rank andlinear independence .The

**exterior algebra**(also known as the**Grassmann algebra**, afterHermann Grassmann [*harvcoltxt|Grassmann|1844 introduced these as "extended" algebras (cf. harvnb|Clifford|1878). He used the word "äußere" (literally translated as "outer", or "exterior") only to indicate the "produkt" he defined, which is nowadays conventionally called "exterior product", probably to distinguish it from the "*] ) of a givenouter product " as defined in modernlinear algebra .vector space "V" over a field "K" is the algebra generated by the exterior product. It is widely used in contemporarygeometry , especiallydifferential geometry andalgebraic geometry through the algebra ofdifferential form s, as well as inmultilinear algebra and related fields.Formally, the exterior algebra is a certain

unital associative algebra over the field "K", containing "V" as a subspace. It is denoted by Λ("V") or Λ^{•}("V") and its multiplication is also known as the "wedge product" or the "exterior product" and is written as $wedge$. The wedge product is anassociative and bilinear operation::$wedge:\; Lambda(V)\; imes\; Lambda(V)\; oLambda(V).$:::$(alpha,eta)mapsto\; alphawedgeeta.$

Its essential feature is that it is "alternating" on "V"::(1) $vwedge\; v\; =\; 0\; mbox\{\; for\; all\; \}vin\; V,$which implies in particular:(2) $uwedge\; v\; =\; -\; vwedge\; u$ for all $u,vin\; V$, and:(3) $v\_1wedge\; v\_2wedgecdots\; wedge\; v\_k\; =\; 0$ whenever $v\_1,\; ldots,\; v\_k\; in\; V$ are linearly dependent. [

*Note that, unlike associativity and bilinearity which are required for all elements of the algebra Λ("V"), these last three properties are imposed only on the algebra's subspace "V". The defining property (1) and property (3) are equivalent; properties (1) and (2) are equivalent unless the characteristic of "K" is two.*]In terms of

category theory , the exterior algebra is a type offunctor on vector spaces, given by a universal construction. The universal construction allows the exterior algebra to be defined, not just for vector spaces over a field, but also for modules over acommutative ring , and for other structures of interest. The exterior algebra is one example of abialgebra , meaning that itsdual space also possesses a product, and this dual product is compatible with the wedge product. This dual algebra is precisely the algebra ofalternating multilinear form s on "V", and the pairing between the exterior algebra and its dual is given by theinterior product .**Motivating examples****Areas in the plane**s

:$\{mathbf\; e\}\_1\; =\; (1,0),quad\; \{mathbf\; e\}\_2\; =\; (0,1).$

Suppose that

:$\{mathbf\; v\}\; =\; v\_1\{mathbf\; e\}\_1\; +\; v\_2\{mathbf\; e\}\_2,\; quad\; \{mathbf\; w\}\; =\; w\_1\{mathbf\; e\}\_1\; +\; w\_2\{mathbf\; e\}\_2$

are a pair of given vectors in

**R**^{2}, written in components. There is a unique parallelogram having**v**and**w**as two of its sides. The "area" of this parallelogram is given by the standarddeterminant formula::$A\; =\; left|detegin\{bmatrix\}\{mathbf\; v\}\; \{mathbf\; w\}end\{bmatrix\}\; ight|\; =\; |v\_1w\_2\; -\; v\_2w\_1|.$

Consider now the exterior product of

**v**and**w**::$\{mathbf\; v\}wedge\; \{mathbf\; w\}\; =\; (v\_1\{mathbf\; e\}\_1\; +\; v\_2\{mathbf\; e\}\_2)wedge\; (w\_1\{mathbf\; e\}\_1\; +\; w\_2\{mathbf\; e\}\_2)=v\_1w\_1\{mathbf\; e\}\_1wedge\{mathbf\; e\}\_1+\; v\_1w\_2\{mathbf\; e\}\_1wedge\; \{mathbf\; e\}\_2+v\_2w\_1\{mathbf\; e\}\_2wedge\; \{mathbf\; e\}\_1+v\_2w\_2\{mathbf\; e\}\_2wedge\; \{mathbf\; e\}\_2$

:$=(v\_1w\_2-v\_2w\_1)\{mathbf\; e\}\_1wedge\{mathbf\; e\}\_2$

where the first step uses the distributive law for the wedge product, and the last uses the fact that the wedge product is alternating. Note that the coefficient in this last expression is precisely the determinant of the matrix [

**v****w**] . The fact that this may be positive or negative has the intuitive meaning that**v**and**w**may be oriented in a counterclockwise or clockwise sense as the vertices of the parallelogram they define. Such an area is called the**signed area**of the parallelogram: the absolute value of the signed area is the ordinary area, and the sign determines its orientation.The fact that this coefficient is the signed area is not an accident. In fact, it is relatively easy to see that the exterior product should be related to the signed area if one tries to axiomatize this area as an algebraic construct. In detail, if A(

**v**,**w**) denotes the signed area of the parallelogram determined by the pair of vectors**v**and**w**, then A must satisfy the following properties:

# A("a**"v**,"b**"w**) = "a b" A(**v**,**w**) for any real numbers "a" and "b", since rescaling either of the sides rescales the area by the same amount (and reversing the direction of one of the sides reverses the orientation of the parallelogram).

# A(**v**,**v**) = 0, since the area of the degenerate parallelogram determined by**v**(i.e., aline segment ) is zero.

# A(**w**,**v**) = −A(**v**,**w**), since interchanging the roles of**v**and**w**reverses the orientation of the parallelogram.

# A(**v**+ "a**"w**,**w**) = A(**v**,**w**), since adding a multiple of**w**to**v**affects neither the base nor the height of the parallelogram and consequently preserves its area.

# A(**e**_{1},**e**_{2}) = 1, since the area of the unit square is one.With the exception of the last property, the wedge product satisfies the same formal properties as the area. In a certain sense, the wedge product generalizes the final property by allowing the area of a parallelogram to be compared to that of any "standard" chosen parallelogram. In other words, the exterior product in two-dimensions is a "basis-independent" formulation of area. [*This axiomatization of areas is due to*]Leopold Kronecker andKarl Weierstrass ; see harvtxt|Bourbaki|1989|loc=Historical Note. For a modern treatment, see harvtxt|MacLane|Birkhoff|1999|loc=Theorem IX.2.2. For an elementary treatment, see harvtxt|Strang|1993|loc=Chapter 5.**Cross and triple products**For vectors in

**R**^{3}, the exterior algebra is closely related to thecross product andtriple product . Using the standard basis {**e**_{1},**e**_{2},**e**_{3}}, the wedge product of a pair of vectors:$mathbf\{u\}\; =\; u\_1\; mathbf\{e\}\_1\; +\; u\_2\; mathbf\{e\}\_2\; +\; u\_3\; mathbf\{e\}\_3$

and

:$mathbf\{v\}\; =\; v\_1\; mathbf\{e\}\_1\; +\; v\_2\; mathbf\{e\}\_2\; +\; v\_3\; mathbf\{e\}\_3$

is

:$mathbf\{u\}\; wedge\; mathbf\{v\}\; =\; (u\_1\; v\_2\; -\; u\_2\; v\_1)\; (mathbf\{e\}\_1\; wedge\; mathbf\{e\}\_2)\; +\; (u\_1\; v\_3\; -\; u\_3\; v\_1)\; (mathbf\{e\}\_1\; wedge\; mathbf\{e\}\_3)\; +\; (u\_2\; v\_3\; -\; u\_3\; v\_2)\; (mathbf\{e\}\_2\; wedge\; mathbf\{e\}\_3)$

where {

**e**_{1}Λ**e**_{2},**e**_{1}Λ**e**_{3},**e**_{2}Λ**e**_{3}} is the basis for the three-dimensional space Λ^{2}(**R**^{3}). This imitates the usual definition of thecross product of vectors in three dimensions.Bringing in a third vector

:$mathbf\{w\}\; =\; w\_1\; mathbf\{e\}\_1\; +\; w\_2\; mathbf\{e\}\_2\; +\; w\_3\; mathbf\{e\}\_3,$

the wedge product of three vectors is

:$mathbf\{u\}\; wedge\; mathbf\{v\}\; wedge\; mathbf\{w\}\; =\; (u\_1\; v\_2\; w\_3\; +\; u\_2\; v\_3\; w\_1\; +\; u\_3\; v\_1\; w\_2\; -\; u\_1\; v\_3\; w\_2\; -\; u\_2\; v\_1\; w\_3\; -\; u\_3\; v\_2\; w\_1)\; (mathbf\{e\}\_1\; wedge\; mathbf\{e\}\_2\; wedge\; mathbf\{e\}\_3)$

where

**e**_{1}Λ**e**_{2}Λ**e**_{3}is the basis vector for the one-dimensional space Λ^{3}(**R**^{3}). This imitates the usual definition of thetriple product .The cross product and triple product in three dimensions each admit both geometric and algebraic interpretations. The cross product

**u**×**v**can be interpreted as a vector which is perpendicular to both**u**and**v**and whose magnitude is equal to the area of the parallelogram determined by the two vectors. It can also be interpreted as the vector consisting of the minors of the matrix with columns**u**and**v**. The triple product of**u**,**v**, and**w**is geometrically a (signed) volume. Algebraically, it is the determinant of the matrix with columns**u**,**v**, and**w**. The exterior product in three-dimensions allows for similar interpretations. In fact, in the presence of a positively orientedorthonormal basis , the exterior product generalizes these notions to higher dimensions.**Formal definitions and algebraic properties**The exterior algebra Λ("V") over a vector space "V" is defined as the

quotient algebra of thetensor algebra by the two-sided ideal "I" generated by all elements of the form $x\; otimes\; x$ such that "x" ∈ "V". [*This definition is a standard one. See, for instance, harvtxt|MacLane|Birkhoff|1999.*] Symbolically,:$Lambda(V)\; :=\; T(V)/I.,$

The wedge product ∧ of two elements of Λ("V") is defined by

:$alphawedgeeta\; =\; alphaotimeseta\; pmod\; I.$

**Anticommutativity of the wedge product**This product is

anticommutative on elements of "V", for supposing that "x", "y" ∈ "V",:$0\; equiv\; (x+y)wedge\; (x+y)\; =\; xwedge\; x\; +\; xwedge\; y\; +\; ywedge\; x\; +\; ywedge\; y\; equiv\; xwedge\; y\; +\; ywedge\; x\; pmod\; I$

whence

:$xwedge\; y\; =\; -\; ywedge\; x.$

More generally, if "x"

_{1}, "x"_{2}, ..., "x"_{k}are elements of "V", and σ is a permutation of the integers [1,...,"k"] , then:$x\_\{sigma(1)\}wedge\; x\_\{sigma(2)\}wedgedotswedge\; x\_\{sigma(k)\}\; =\; \{\; m\; sgn\}(sigma)x\_1wedge\; x\_2wedgedots\; wedge\; x\_k,$

where sgn(σ) is the signature of the permutation σ. [

*A proof of this can be found in more generality in harvtxt|Bourbaki|1989.*]**The exterior power**The "k"th

**exterior power**of "V", denoted Λ^{"k"}("V"), is thevector subspace of Λ("V") spanned by elements of the form:$x\_1wedge\; x\_2wedgedotswedge\; x\_k,quad\; x\_iin\; V,\; i=1,2,dots,\; k.$If α ∈ Λ

^{"k"}("V"), then α is said to be a "k"-**multivector**. If, furthermore, α can be expressed as a wedge product of "k" elements of "V", then α is said to be**decomposable**. Although decomposable multivectors span Λ^{"k"}("V"), not every element of Λ^{"k"}("V") is decomposable. For example, in**R**^{4}, the following 2-multivector is not decomposable::$alpha\; =\; e\_1wedge\; e\_2\; +\; e\_3wedge\; e\_4.$(This is in fact asymplectic form , since α ∧ α ≠ 0. [*See harvtxt|Sternberg|1964|loc=§III.6.*] )**Basis and dimension**If the dimension of "V" is "n" and {"e"

_{1},...,"e"_{"n"}} is a basis of "V", then the set:$\{e\_\{i\_1\}wedge\; e\_\{i\_2\}wedgecdotswedge\; e\_\{i\_k\}\; mid\; 1le\; i\_1\; <\; i\_2\; <\; cdots\; <\; i\_k\; le\; n\}$is a basis for Λ^{"k"}("V"). The reason is the following: given any wedge product of the form:$v\_1wedgecdotswedge\; v\_k$then every vector "v"_{"j"}can be written as alinear combination of the basis vectors "e"_{"i"}; using the bilinearity of the wedge product, this can be expanded to a linear combination of wedge products of those basis vectors. Any wedge product in which the same basis vector appears more than once is zero; any wedge product in which the basis vectors do not appear in the proper order can be reordered, changing the sign whenever two basis vectors change places. In general, the resulting coefficients of the basis "k"-vectors can be computed as the minors of the matrix that describes the vectors "v"_{"j"}in terms of the basis "e"_{"i"}.By counting the basis elements, the dimension of Λ

^{"k"}("V") is thebinomial coefficient C("n","k"). In particular, Λ^{"k"}("V") = {0} for "k" > "n".Any element of the exterior algebra can be written as a sum of multivectors. Hence, as a vector space the exterior algebra is a

direct sum :$Lambda(V)\; =\; Lambda^0(V)oplus\; Lambda^1(V)\; oplus\; Lambda^2(V)\; oplus\; cdots\; oplus\; Lambda^n(V)$(where by convention Λ^{0}("V") = "K" and Λ^{1}("V") = "V"), and therefore its dimension is equal to the sum of the binomial coefficients, which is 2^{"n"}.**Rank of a multivector**If α ∈ Λ

^{"k"}("V"), then it is possible to express α as a linear combination of decomposable multivectors::$alpha\; =\; alpha^\{(1)\}\; +\; alpha^\{(2)\}\; +\; cdots\; +\; alpha^\{(s)\}$

where each α

^{("i")}is decomposable, say:$alpha^\{(i)\}\; =\; alpha^\{(i)\}\_1wedgecdotswedgealpha^\{(i)\}\_k,quad\; i=1,2,dots,\; s.$

The

**rank**of the multivector α is the minimal number of decomposable multivectors in such an expansion of α. This is similar to the notion oftensor rank .Rank is particularly important in the study of 2-multivectors harv|Sternberg|1974|loc=§III.6 harv|Bryant|Chern|Gardner|Goldschmidt|1991. The rank of a 2-multivector α can be identified with the rank of the matrix of coefficients of α in a basis. Thus if "e"

_{"i"}is a basis for "V", then α can be expressed uniquely as:$alpha\; =\; sum\_\{i,j\}a\_\{ij\}e\_iwedge\; e\_j$

where "a"

_{"ij"}= −"a"_{"ji"}(the matrix of coefficients isskew-symmetric ). The rank of α agrees with the rank of the matrix "a"_{"ij"}.In characteristic 0, the 2-multivector α has rank "p" if and only if

:$underset\{p\}\{underbrace\{alphawedgecdotswedgealpha\; ot=\; 0$

and

:$underset\{p+1\}\{underbrace\{alphawedgecdotswedgealpha\; =\; 0.$

**Graded structure**The wedge product of a "k"-multivector with a "p"-multivector is a ("k"+"p")-multivector, once again invoking bilinearity. As a consequence, the direct sum decomposition of the preceding section

:$Lambda(V)\; =\; Lambda^0(V)oplus\; Lambda^1(V)\; oplus\; Lambda^2(V)\; oplus\; cdots\; oplus\; Lambda^n(V)$

gives the exterior algebra the additional structure of a

graded algebra . Symbolically,:$left(Lambda^k(V)\; ight)wedgeleft(Lambda^p(V)\; ight)sub\; Lambda^\{k+p\}(V).$

Moreover, the wedge product is graded anticommutative, meaning that if α ∈ Λ

^{k}("V") and β ∈ Λ^{p}("V"), then:$alphawedgeeta\; =\; (-1)^\{kp\}etawedgealpha.$

In addition to studying the graded structure on the exterior algebra, harvtxt|Bourbaki|1989 studies additional graded structures on exterior algebras, such as those on the exterior algebra of a

graded module (a module that already carries its own gradation).**Universal property**Let "V" be a vector space over the field "K". Informally, multiplication in Λ("V") is performed by manipulating symbols and imposing a

distributive law , anassociative law , and using the identities "v" ∧ "v" = 0 for "v" ∈ "V" and "v" ∧ "w" = -"w" ∧ "v" for "v", "w" ∈ "V". Formally, Λ("V") is the "most general" algebra in which these rules hold for the multiplication, in the sense that any unital associative "K"-algebra containing "V" with alternating multiplication on "V" must contain a homomorphic image of Λ("V"). In other words, the exterior algebra has the followinguniversal property : [*See harvtxt|Bourbaki|1989|loc=III.7.1, and harvtxt|MacLane|Birkhoff|1999|loc=Theorem XVI.6.8. More detail on universal properties in general can be found in harvtxt|MacLane|Birkhoff|1999|loc=Chapter VI, and throughout the works of Bourbaki.*]Given any unital associative "K"-algebra "A" and any "K"-linear map "j" : "V" → "A" such that "j"("v")"j"("v") = 0 for every "v" in "V", then there exists "precisely one" unitalalgebra homomorphism "f" : Λ("V") → "A" such that "f"("v") = "j"("v") for all "v" in "V".To construct the most general algebra that contains "V" and whose multiplication is alternating on "V", it is natural to start with the most general algebra that contains "V", the

tensor algebra "T"("V"), and then enforce the alternating property by taking a suitable quotient. We thus take the two-sided ideal "I" in "T"("V") generated by all elements of the form "v"⊗"v" for "v" in "V", and define Λ("V") as the quotient:Λ("V") = T("V")/"I"

(and use Λ as the symbol for multiplication in Λ("V")). It is then straightforward to show that Λ("V") contains "V" and satisfies the above universal property.

As a consequence of this construction, the operation of assigning to a vector space "V" its exterior algebra Λ("V") is a

functor from the category of vector spaces to the category of algebras.Rather than defining Λ("V") first and then identifying the exterior powers Λ

^{"k"}("V") as certain subspaces, one may alternatively define the spaces Λ^{"k"}("V") first and then combine them to form the algebra Λ("V"). This approach is often used in differential geometry and is described in the next section.**Generalizations**Given a

commutative ring "R" and an "R"-module "M", we can define the exterior algebra Λ("M") just as above, as a suitable quotient of the tensor algebra**T**("M"). It will satisfy the analogous universal property. Many of the properties of Λ("M") also require that "M" be aprojective module . Where finite-dimensionality is used, the properties further require that "M" be finitely generated and projective. Generalizations to the most common situations can be found in harv|Bourbaki|1989.Exterior algebras of

vector bundle s are frequently considered in geometry and topology. There are no essential differences between the algebraic properties of the exterior algebra of finite-dimensional vector bundles and those of the exterior algebra of finitely-generated projective modules, by theSerre-Swan theorem . More general exterior algebras can be defined for sheaves of modules.**Duality****Alternating operators**Given two vector spaces "V" and "X", an

**alternating operator**(or "anti-symmetric operator") from "V"^{"k"}to "X" is amultilinear map:"f": "V"

^{"k"}→ "X "such that whenever "v"

_{1},...,"v"_{"k"}arelinearly dependent vectors in "V", then:"f"("v"_{1},...,"v"_{"k"}) = 0.The most famous example is the

determinant , an alternating operator from ("K"^{"n"})^{"n"}to "K".The

^{"k"}→ Λ^{"k"}("V")which associates to "k" vectors from "V" their wedge product, i.e. their corresponding "k"-vector, is also alternating. In fact, this map is the "most general" alternating operator defined on "V"^{"k"}: given any other alternating operator "f" : "V"^{"k"}→ "X", there exists a uniquelinear map φ: Λ^{"k"}("V") → "X" with "f" = φ o "w". Thisuniversal property characterizes the space Λ^{"k"}("V") and can serve as its definition.**Alternating multilinear forms**The above discussion specializes to the case when "X" = "K", the base field. In this case an alternating multilinear function:"f" : "V"

^{k}→ "K"is called an**alternating multilinear form**. The set of all alternating multilinear forms is a vector space, as the sum of two such maps, or the multiplication of such a map with a scalar, is again alternating. If "V" has finite dimension "n", then this space can be identified with Λ^{"k"}("V"^{∗}), where "V"^{∗}denotes thedual space of "V". In particular, the dimension of the space of anti-symmetric maps from "V"^{"k"}to "K" is thebinomial coefficient "n" choose "k".Under this identification, the wedge product takes a concrete form: it produces a new anti-symmetric map from two given ones. Suppose ω : "V"

^{"k"}→ "K" and η : "V"^{"m"}→ "K" are two anti-symmetric maps. As in the case oftensor product s of multilinear maps, the number of variables of their wedge product is the sum of the numbers of their variables. It is defined as follows::$omegawedgeeta=frac\{(k+m)!\}\{k!,m!\}\{\; m\; Alt\}(omegaotimeseta)$

where the alternation Alt of a multilinear map is defined to be the signed average of the values over all the

permutation s of its variables::$\{\; m\; Alt\}(omega)(x\_1,ldots,x\_k)=frac\{1\}\{k!\}sum\_\{sigmain\; S\_k\}\{\; m\; sgn\}(sigma),omega(x\_\{sigma(1)\},ldots,x\_\{sigma(k)\})$

This definition of the wedge product is well-defined even if the fields "K" has finite characteristic, ifone considers an equivalent version of the above that does not use factorials or any constants:

:$omega\; wedge\; eta(x\_1,ldots,x\_\{k+m\})\; =\; sum\_\{sigma\; in\; Sh\_\{k,m\; \{\; m\; sgn\}(sigma),omega(x\_\{sigma(1)\},\; ldots,\; x\_\{sigma(k)\})\; eta(x\_\{sigma(k+1)\},\; ldots,\; x\_\{sigma(k+m)\}),$

where here "Sh"

_{"k","m"}⊂ "S"_{"k"+"m"}is the subset of ("k,m") shuffles:permutation s σ of the set {1,2,…,"k"+"m"} such that σ(1) < σ(2) < … < σ("k"), and σ("k"+1) < σ("k"+2)< … <σ("k"+"m"). [*Some conventions, particularly in physics, define the wedge product as**:$omegawedgeeta=\{\; m\; Alt\}(omegaotimeseta).$**This convention is not adopted here, but is discussed in connection with alternating tensors.*]**Bialgebra structure**In formal terms, there is a correspondence between the graded dual of the graded algebra Λ("V") and alternating multilinear forms on "V". The wedge product of multilinear forms defined above is dual to a

coproduct defined on Λ("V"), giving the structure of acoalgebra .The

**coproduct**is a linear function Δ : Λ("V") → Λ("V") ⊗ Λ("V") given on decomposable elements by:$Delta(x\_1wedgedotswedge\; x\_k)\; =\; sum\_\{p=0\}^k\; sum\_\{sigmain\; Sh\_\{p,k-p\; \{\; m\; sgn\}(sigma)\; (x\_\{sigma(1)\}wedgedotswedge\; x\_\{sigma(p)\})otimes\; (x\_\{sigma(p+1)\}wedgedotswedge\; x\_\{sigma(k)\}).$For example,:$Delta(x\_1)\; =\; 1\; otimes\; x\_1\; +\; x\_1\; otimes\; 1,$

:$Delta(x\_1\; wedge\; x\_2)\; =\; 1\; otimes\; (x\_1\; wedge\; x\_2)\; +\; x\_1\; otimes\; x\_2\; -\; x\_2\; otimes\; x\_1\; +\; (x\_1\; wedge\; x\_2)\; otimes\; 1.$

This extends by linearity to an operation defined on the whole exterior algebra. In terms of the coproduct, the wedge product on the dual space is just the graded dual of the coproduct:

:$(alphawedgeeta)(x\_1wedgedotswedge\; x\_k)\; =\; (alphaotimeseta)left(Delta(x\_1wedgedotswedge\; x\_k)\; ight)$

where the tensor product on the right-hand side is of multilinear linear maps (extended by zero on elements of incompatible homogeneous degree: more precisely, α∧β = ε o (α⊗β) o Δ, where ε is the counit, as defined presently).

The

**counit**is the homomorphism ε : Λ("V") → "K" which returns the 0-graded component of its argument. The coproduct and counit, along with the wedge product, define the structure of abialgebra on the exterior algebra.**The interior product**Suppose that "V" is finite-dimensional. If "V*" denotes the

dual space to the vector space "V", then for each α ∈ "V"^{*}, it is possible to define an antiderivation on the algebra Λ(V),:$i\_alpha:Lambda^k\; V\; ightarrowLambda^\{k-1\}V.$

This derivation is called the

**interior product**with α, or sometimes the**insertion operator**, or**contraction**by α.Suppose that

**w**∈ Λ^{k}"V". Then**w**is a multilinear mapping of "V"^{*}to**R**, so it is defined by its values on the "k"-foldCartesian product "V"^{*}× "V"^{*}× ... × "V"^{*}. If "u"_{1}, "u"_{2}, ..., "u"_{k-1}are "k-1" elements of "V"^{*}, then define:$(i\_alpha\; \{old\; w\})(u\_1,u\_2dots,u\_\{k-1\})=\{old\; w\}(alpha,u\_1,u\_2,dots,\; u\_\{k-1\}).$

Additionally, let "i"

_{α}"f" = 0 whenever "f" is a pure scalar (i.e., belonging to Λ^{0}"V").**Axiomatic characterization and properties**The interior product satisfies the following properties:

# For each "k" and each α ∈ V

^{*},

#::$i\_alpha:Lambda^kV\; ightarrow\; Lambda^\{k-1\}V.$

#:(By convention, Λ^{−1}= 0.)

# If "v" is an element of "V" ( = Λ^{1}"V"), then "i"_{α}"v" = α("v") is the dual pairing between elements of "V" and elements of "V"^{*}.

# For each α ∈ "V"^{*}, "i"_{α is a graded derivation of degree −1:#::$i\_alpha\; (awedge\; b)\; =\; (i\_alpha\; a)wedge\; b\; +\; (-1)^\{deg\; a\}awedge\; (i\_alpha\; b).$}In fact, these three properties are sufficient to characterize the interior product as well as define it in the general infinite-dimensional case.

Further properties of the interior product include::* $i\_alphacirc\; i\_alpha\; =\; 0.$:* $i\_alphacirc\; i\_eta\; =\; -i\_etacirc\; i\_alpha.$

**Hodge duality**Suppose that "V" has finite dimension "n". Then the interior product induces a canonical isomorphism of vector spaces:$Lambda^k(V^*)\; otimes\; Lambda^n(V)\; o\; Lambda^\{n-k\}(V).$In the geometrical setting, a non-zero element of the top exterior power Λ

^{n}(V) (which is a one-dimensional vector space) is sometimes called a(orvolume form **orientation form**, although this term may sometimes lead to ambiguity.) Relative to a given volume form σ, the isomorphism is given explicitly by:$alpha\; in\; Lambda^k(V^*)\; mapsto\; i\_alphasigma\; in\; Lambda^\{n-k\}(V).$

If, in addition to a volume form, the vector space "V" is equipped with an

inner product identifying "V" with "V"^{*}, then the resulting isomorphism is called the**Hodge dual**(or more commonly the**Hodge star operator**):$*\; :\; Lambda^k(V)\; ightarrow\; Lambda^\{n-k\}(V).$

The composite of * with itself maps Λ

^{k}("V") → Λ^{k}("V") and is always a scalar multiple of the identity map. In most applications, the volume form is compatible with the inner product in the sense that it is a wedge product of anorthonormal basis of "V". In this case,:$*circ\; *\; :\; Lambda^k(V)\; o\; Lambda^k(V)\; =\; (-1)^\{k(n-k)\; +\; q\}I$where "I" is the identity, and the inner product hasmetric signature ("p","q") — "p" plusses and "q" minuses.**Functoriality**Suppose that "V" and "W" are a pair of vector spaces and "f" : "V" → "W" is a

linear transformation . Then, by the universal construction, there exists a unique homomorphism of graded algebras:$Lambda(f)\; :\; Lambda(V)\; ightarrow\; Lambda(W)$

such that

:$Lambda(f)|\_\{Lambda^1(V)\}\; =\; f\; :\; V=Lambda^1(V)\; ightarrow\; W=Lambda^1(W).$

In particular, Λ("f") preserves homogeneous degree. The "k"-graded components of Λ("f") are given on decomposable elements by:$Lambda(f)(x\_1wedge\; dots\; wedge\; x\_k)\; =\; f(x\_1)wedgedotswedge\; f(x\_k).$

Let

:$Lambda^k(f)\; =\; Lambda(f)\_\{Lambda^k(V)\}\; :\; Lambda^k(V)\; ightarrow\; Lambda^k(W).$

The components of the transformation Λ("k") relative to a basis of "V" and "W" is the matrix of "k" × "k" minors of "f". In particular, if "V" = "W" and "V" is of finite dimension "n", then Λ

^{"n"}("f") is a mapping of a one-dimensional vector space Λ^{"n"}to itself, and is therefore given by a scalar: thedeterminant of "f".**Exactness**If

:$0\; ightarrow\; U\; ightarrow\; V\; ightarrow\; W\; ightarrow\; 0$

is a

short exact sequence of vector spaces, then:$0\; o\; Lambda^1(U)wedgeLambda(V)\; o\; Lambda(V)\; ightarrow\; Lambda(W)\; ightarrow\; 0$

is an exact sequence of graded vector spaces [

*This part of the statement also holds in greater generality if "V" and "W" are modules over a commutative ring: That Λ converts epimorphisms to epimorphisms. See harvtxt|Bourbaki|1989|loc=Proposition 3, III.7.2.*] as is:$0\; o\; Lambda(U)\; oLambda(V).$ [*This statement generalizes only to the case where "V" and "W" are projective modules over a commutative ring. Otherwise, it is generally not the case that Λ converts monomorphisms to monomorphisms. See harvtxt|Bourbaki|1989|loc=Corollary to Proposition 12, III.7.9.*]**Direct sums**In particular, the exterior algebra of a direct sum is isomorphic to the tensor product of the exterior algebras:

:$Lambda(Voplus\; W)=\; Lambda(V)otimesLambda(W).$

This is a graded isomorphism; i.e.,

:$Lambda^k(Voplus\; W)=\; igoplus\_\{p+q=k\}\; Lambda^p(V)otimesLambda^q(W).$

Slightly more generally, if

:$0\; ightarrow\; U\; ightarrow\; V\; ightarrow\; W\; ightarrow\; 0$

is a

short exact sequence of vector spaces then $Lambda^k(V)$ has a filtration:$0\; =\; F^0\; subseteq\; F^1\; subseteq\; dotsb\; subseteq\; F^k\; subseteq\; F^\{k+1\}\; =\; Lambda^k(V)$

with quotients $F^\{p+1\}/F^p\; =\; Lambda^\{k-p\}(U)\; otimes\; Lambda^p(W)$. In particular, if "U" is 1-dimensional then

:$0\; ightarrow\; U\; otimes\; Lambda^\{k-1\}(W)\; ightarrow\; Lambda^k(V)\; ightarrow\; Lambda^k(W)\; ightarrow\; 0$

is exact, and if "W" is 1-dimensional then

:$0\; ightarrow\; Lambda^k(U)\; ightarrow\; Lambda^k(V)\; ightarrow\; Lambda^\{k-1\}(U)\; otimes\; W\; ightarrow\; 0$

is exact. [

*Such a filtration also holds for*]vector bundle s, and projective modules over a commutative ring. This is thus more general than the result quoted above for direct sums, since not every short exact sequence splits in other abelian categories.**The alternating tensor algebra**If "K" is a field of characteristic 0, [

*See harvtxt|Bourbaki|1989|loc=III.7.5 for generalizations.*] then the exterior algebra of a vector space "V" can be canonically identified with the vector subspace of T("V") consisting ofantisymmetric tensor s. Recall that the exterior algebra is the quotient of T("V") by the ideal "I" generated by "x" ⊗ "x".Let T

^{r}("V") be the space of homogeneous tensors of degree "r". This is spanned by decomposable tensors:$v\_1otimesdotsotimes\; v\_r,quad\; v\_iin\; V.$

The

**antisymmetrization**(or sometimes the**skew-symmetrization**) of a decomposable tensor is defined by:$ext\{Alt\}(v\_1otimesdotsotimes\; v\_r)\; =\; frac\{1\}\{r!\}sum\_\{sigmainmathfrak\{S\}\_r\}\; \{\; m\; sgn\}(sigma)\; v\_\{sigma(1)\}otimesdotsotimes\; v\_\{sigma(r)\}$

where the sum is taken over the

symmetric group of permutations on the symbols {1,...,"r"}. This extends by linearity and homogeneity to an operation, also denoted by Alt, on the full tensor algebra T("V"). The image Alt(T("V")) is the**alternating tensor algebra**, denoted A("V"). This is a vector subspace of T("V"), and it inherits the structure of a graded vector space from that on T("V"). It carries an associative graded product $widehat\{otimes\}$ defined by:$t\; widehat\{otimes\}\; s\; =\; ext\{Alt\}(totimes\; s).$

Although this product differs from the tensor product, the kernel of "Alt" is precisely the ideal "I" (again, assuming that "K" has characteristic 0), and there is a canonical isomorphism

:$A(V)cong\; Lambda(V).$

**Index notation**Suppose that "V" has finite dimension "n", and that a basis

**e**_{1}, ...,**e**_{"n"}of "V" is given. then any alternating tensor "t" ∈ A^{"r"}("V") ⊂ "T"^{"r"}("V") can be written inindex notation as:$t\; =\; t^\{i\_1i\_2dots\; i\_r\},\; \{mathbf\; e\}\_\{i\_1\}otimes\; \{mathbf\; e\}\_\{i\_2\}otimesdotsotimes\; \{mathbf\; e\}\_\{i\_r\}$

where "t"

^{"i"1 ... "i""r"}is completely antisymmetric in its indices.The wedge product of two alternating tensors "t" and "s" of ranks "r" and "p" is given by

:$twidehat\{otimes\}\; s\; =\; frac\{1\}\{(r+p)!\}sum\_\{sigmain\; \{mathfrak\; S\}\_\{r+p\; ext\{sgn\}(sigma)t^\{i\_\{sigma(1)\}dots\; i\_\{sigma(r)s^\{i\_\{sigma(r+1)\}dots\; i\_\{sigma(r+p)\; \{mathbf\; e\}\_\{i\_1\}otimes\; \{mathbf\; e\}\_\{i\_2\}otimesdotsotimes\; \{mathbf\; e\}\_\{i\_\{r+p.$

The components of this tensor are precisely the skew part of the components of the tensor product "s" ⊗ "t", denoted by square brackets on the indices:

:$(twidehat\{otimes\}\; s)^\{i\_1dots\; i\_\{r+p\; =\; t^\{\; [i\_1dots\; i\_r\}s^\{i\_\{r+1\}dots\; i\_\{r+p\}]\; \}.$

The interior product may also be described in index notation as follows. Let $t\; =\; t^\{i\_0i\_1dots\; i\_\{r-1$ be an antisymmetric tensor of rank "r". Then, for α ∈ "V"

^{*}, "i"_{α}**t**is an alternating tensor of rank "r"-1, given by:$(i\_alpha\; t)^\{i\_1dots\; i\_\{r-1=rsum\_\{j=0\}^nalpha\_j\; t^\{ji\_1dots\; i\_\{r-1.$

where "n" is the dimension of "V".

**Applications****Linear geometry**The decomposable "k"-vectors have geometric interpretations: the bivector $uwedge\; v$ represents the plane spanned by the vectors, "weighted" with a number, given by the area of the oriented

parallelogram with sides "u" and "v". Analogously, the 3-vector $uwedge\; vwedge\; w$ represents the spanned 3-space weighted by the volume of the orientedparallelepiped with edges "u", "v", and "w".**Projective geometry**Decomposable "k"-vectors in Λ

^{"k"}"V" correspond to weighted "k"-dimensionalsubspace s of "V". In particular, theGrassmannian of "k"-dimensional subspaces of "V", denoted "Gr"_{k}("V"), can be naturally identified with an algebraic subvariety of theprojective space **P**(Λ^{k}"V"). This is called thePlücker embedding .**Differential geometry**The exterior algebra has notable applications in

differential geometry , where it is used to definedifferential form s. Adifferential form at a point of adifferentiable manifold is an alternating multilinear form on thetangent space at the point. Equivalently, a differential form of degree "k" is alinear functional on the "k"-th exterior power of the tangent space. As a consequence, the wedge product of multilinear forms defines a natural wedge product for differential forms. Differential forms play a major role in diverse areas of differential geometry.In particular, the

exterior derivative gives the exterior algebra of differential forms on a manifold the structure of adifferential algebra . The exterior derivative commutes with pullback along smooth mappings between manifolds, and it is therefore a naturaldifferential operator . The exterior algebra of differential forms, equipped with the exterior derivative, is a differential complex whose cohomology is called thede Rham cohomology of the underlying manifold and plays a vital role in thealgebraic topology of differentiable manifolds.**Representation theory**In

representation theory , the exterior algebra is one of the two fundamentalSchur functor s on the category of vector spaces, the other being thesymmetric algebra . Together, these constructions are used to generate theirreducible representation s of thegeneral linear group .**Physics**The exterior algebra is an archetypal example of a

superalgebra , which plays a fundamental role in physical theories pertaining tofermion s andsupersymmetry . For a physical discussion, seeGrassmann number . For various other applications of related ideas to physics, seesuperspace andsupergroup (physics) .**History**The exterior algebra was first introduced by

Hermann Grassmann in 1844 under the blanket term of "Ausdehnungslehre", or "Theory of Extension". [*harvcoltxt|Kannenberg|2000 published a translation of Grassmann's work in English; he translated "Ausdehnungslehre" as "Extension Theory".*] This referred more generally to an algebraic (or axiomatic) theory of extended quantities and was one of the early precursors to the modern notion of avector space . Saint-Venant also published similar ideas of exterior calculus for which he claimed priority over Grassmann. [*J Itard, Biography in Dictionary of Scientific Biography (New York 1970-1990).*]The algebra itself was built from a set of rules, or axioms, capturing the formal aspects of Cayley and Sylvester's theory of

multivector s. It was thus a "calculus", much like thepropositional calculus , except focused exclusively on the task of formal reasoning in geometrical terms. [*Authors have in the past referred to this calculus variously as the "calculus of extension" (harvnb|Whitehead|1898; harvnb|Forder|1941), or "extensive algebra" harv|Clifford|1878, and recently as "extended vector algebra" harv|Browne|2007.*] In particular, this new development allowed for an "axiomatic" characterization of dimension, a property that had previously only been examined from the coordinate point of view.The import of this new theory of vectors and multivectors was lost to mid 19th century mathematicians, [

*harvnb|Bourbaki|1989|p=661.*] until being thoroughly vetted byGiuseppe Peano in 1888. Peano's work also remained somewhat obscure until the turn of the century, when the subject was unified by members of the French geometry school (notablyHenri Poincaré ,Élie Cartan , andGaston Darboux ) who applied Grassmann's ideas to the calculus ofdifferential form s.A short while later,

Alfred North Whitehead , borrowing from the ideas of Peano and Grassmann, introduced hisuniversal algebra . This then paved the way for the 20th century developments ofabstract algebra by placing the axiomatic notion of an algebraic system on a firm logical footing.**ee also***

symmetric algebra , the symmetric analog

*Clifford algebra , a quantum deformation of the exterior algebra by aquadratic form

*multilinear algebra

*tensor algebra

*geometric algebra

*Koszul complex **Notes****References****Mathematical references***:: Includes a treatment of alternating tensors and alternating forms, as well as a detailed discussion of Hodge duality from the perspective adopted in this article.

*:: This is the "main mathematical reference" for the article. It introduces the exterior algebra of a module over a commutative ring (although this article specializes primarily to the case when the ring is a field), including a discussion of the universal property, functoriality, duality, and the bialgebra structure. See chapters III.7 and III.11.

*:: This book contains applications of exterior algebras to problems inpartial differential equations . Rank and related concepts are developed in the early chapters.

*:: Chapter XVI sections 6-10 give a more elementary account of the exterior algebra, including duality, determinants and minors, and alternating forms.

*:: Contains a classical treatment of the exterior algebra as alternating tensors, and applications to differential geometry.**Historical references***

*

*

* (The Linear Extension Theory - A new Branch of Mathematics)

*

* [Geometric Calculus according to Grassmann's Ausdehnungslehre, preceded by the Operations of Deductive Logic]

***Other references and further reading***:: An introduction to the exterior algebra, and

geometric algebra , with a focus on applications. Also includes a history section and bibliography.

*:: Includes applications of the exterior algebra to differential forms, specifically focused on integration andStokes's theorem . The notation Λ^{"k"}"V" in this text is used to mean the space of alternating "k"-forms on "V"; i.e., for Spivak Λ^{"k"}"V" is what this article would call Λ^{"k"}"V"*. Spivak discusses this in Addendum 4.

*:: Includes an elementary treatment of the axiomatization of determinants as signed areas, volumes, and higher-dimensional volumes.

*

* Wendell H. Fleming (1965) "Functions of Several Variables",Addison-Wesley .:: Chapter 6: Exterior algebra and differential calculus, pages 205-38. This textbook inmultivariate calculus introduces the exterior algebra of differential forms adroitly into the calculus sequence for colleges.

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