# Exterior algebra

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Exterior algebra

In mathematics, the exterior product or wedge product of vectors is an algebraic construction generalizing certain features of the cross product to higher dimensions. Like the cross product, and the scalar triple product, the exterior product of vectors is used in Euclidean geometry to study areas, volumes, and their higher-dimensional analogs. In linear algebra, the exterior product provides an abstract algebraic basis-independent manner for describing the determinant and the minors of a linear transformation, and is fundamentally related to ideas of rank and linear independence.

The exterior algebra (also known as the Grassmann algebra, after Hermann 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 "outer product" as defined in modern linear algebra.] ) of a given vector space "V" over a field "K" is the algebra generated by the exterior product. It is widely used in contemporary geometry, especially differential geometry and algebraic geometry through the algebra of differential forms, as well as in multilinear 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 an associative and bilinear operation:

:$wedge: Lambda\left(V\right) imes Lambda\left(V\right) oLambda\left(V\right).$:::

Its essential feature is that it is "alternating" on "V"::(1) $vwedge v = 0 mbox\left\{ for all \right\}vin V,$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 of functor 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 a commutative ring, and for other structures of interest. The exterior algebra is one example of a bialgebra, meaning that its dual space also possesses a product, and this dual product is compatible with the wedge product. This dual algebra is precisely the algebra of alternating multilinear forms on "V", and the pairing between the exterior algebra and its dual is given by the interior product.

Motivating examples

Areas in the plane

s

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

Suppose that

:$\left\{mathbf v\right\} = v_1\left\{mathbf e\right\}_1 + v_2\left\{mathbf e\right\}_2, quad \left\{mathbf w\right\} = w_1\left\{mathbf e\right\}_1 + w_2\left\{mathbf e\right\}_2$

are a pair of given vectors in R2, 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 standard determinant formula:

:

Consider now the exterior product of v and w:

:$\left\{mathbf v\right\}wedge \left\{mathbf w\right\} = \left(v_1\left\{mathbf e\right\}_1 + v_2\left\{mathbf e\right\}_2\right)wedge \left(w_1\left\{mathbf e\right\}_1 + w_2\left\{mathbf e\right\}_2\right)=v_1w_1\left\{mathbf e\right\}_1wedge\left\{mathbf e\right\}_1+ v_1w_2\left\{mathbf e\right\}_1wedge \left\{mathbf e\right\}_2+v_2w_1\left\{mathbf e\right\}_2wedge \left\{mathbf e\right\}_1+v_2w_2\left\{mathbf e\right\}_2wedge \left\{mathbf e\right\}_2$

:$=\left(v_1w_2-v_2w_1\right)\left\{mathbf e\right\}_1wedge\left\{mathbf e\right\}_2$

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., a line 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(e1, e2) = 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 and Karl 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 R3, the exterior algebra is closely related to the cross product and triple product. Using the standard basis {e1, e2, e3}, the wedge product of a pair of vectors

:$mathbf\left\{u\right\} = u_1 mathbf\left\{e\right\}_1 + u_2 mathbf\left\{e\right\}_2 + u_3 mathbf\left\{e\right\}_3$

and

:$mathbf\left\{v\right\} = v_1 mathbf\left\{e\right\}_1 + v_2 mathbf\left\{e\right\}_2 + v_3 mathbf\left\{e\right\}_3$

is

:$mathbf\left\{u\right\} wedge mathbf\left\{v\right\} = \left(u_1 v_2 - u_2 v_1\right) \left(mathbf\left\{e\right\}_1 wedge mathbf\left\{e\right\}_2\right) + \left(u_1 v_3 - u_3 v_1\right) \left(mathbf\left\{e\right\}_1 wedge mathbf\left\{e\right\}_3\right) + \left(u_2 v_3 - u_3 v_2\right) \left(mathbf\left\{e\right\}_2 wedge mathbf\left\{e\right\}_3\right)$

where {e1 Λ e2, e1 Λ e3, e2 Λ e3} is the basis for the three-dimensional space Λ2(R3). This imitates the usual definition of the cross product of vectors in three dimensions.

Bringing in a third vector

:$mathbf\left\{w\right\} = w_1 mathbf\left\{e\right\}_1 + w_2 mathbf\left\{e\right\}_2 + w_3 mathbf\left\{e\right\}_3,$

the wedge product of three vectors is

:$mathbf\left\{u\right\} wedge mathbf\left\{v\right\} wedge mathbf\left\{w\right\} = \left(u_1 v_2 w_3 + u_2 v_3 w_1 + u_3 v_1 w_2 - u_1 v_3 w_2 - u_2 v_1 w_3 - u_3 v_2 w_1\right) \left(mathbf\left\{e\right\}_1 wedge mathbf\left\{e\right\}_2 wedge mathbf\left\{e\right\}_3\right)$

where e1 Λ e2 Λ e3 is the basis vector for the one-dimensional space Λ3(R3). This imitates the usual definition of the triple 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 oriented orthonormal 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 the tensor 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,

:$Lambda\left(V\right) := T\left(V\right)/I.,$

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

:

Anticommutativity of the wedge product

This product is anticommutative on elements of "V", for supposing that "x", "y" ∈ "V",

:$0 equiv \left(x+y\right)wedge \left(x+y\right) = xwedge x + xwedge y + ywedge x + ywedge y equiv xwedge y + ywedge x pmod I$

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

:$x_\left\{sigma\left(1\right)\right\}wedge x_\left\{sigma\left(2\right)\right\}wedgedotswedge x_\left\{sigma\left(k\right)\right\} = \left\{ m sgn\right\}\left(sigma\right)x_1wedge x_2wedgedots wedge x_k,$

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 the vector 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 R4, the following 2-multivector is not decomposable::$alpha = e_1wedge e_2 + e_3wedge e_4.$(This is in fact a symplectic 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:$\left\{e_\left\{i_1\right\}wedge e_\left\{i_2\right\}wedgecdotswedge e_\left\{i_k\right\} mid 1le i_1 < i_2 < cdots < i_k le n\right\}$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 a linear 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 the binomial 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:$Lambda\left(V\right) = Lambda^0\left(V\right)oplus Lambda^1\left(V\right) oplus Lambda^2\left(V\right) oplus cdots oplus Lambda^n\left(V\right)$(where by convention Λ0("V") = "K" and Λ1("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:

:$alpha = alpha^\left\{\left(1\right)\right\} + alpha^\left\{\left(2\right)\right\} + cdots + alpha^\left\{\left(s\right)\right\}$

where each α("i") is decomposable, say

:$alpha^\left\{\left(i\right)\right\} = alpha^\left\{\left(i\right)\right\}_1wedgecdotswedgealpha^\left\{\left(i\right)\right\}_k,quad i=1,2,dots, s.$

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 of tensor 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

:$alpha = sum_\left\{i,j\right\}a_\left\{ij\right\}e_iwedge e_j$

where "a""ij" = −"a""ji" (the matrix of coefficients is skew-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

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

and

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

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

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

gives the exterior algebra the additional structure of a graded algebra. Symbolically,

:$left\left(Lambda^k\left(V\right) ight\right)wedgeleft\left(Lambda^p\left(V\right) ight\right)sub Lambda^\left\{k+p\right\}\left(V\right).$

Moreover, the wedge product is graded anticommutative, meaning that if α ∈ Λk("V") and β ∈ Λp("V"), then

:

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, an associative 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 following universal 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" unital algebra 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 a projective 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 bundles 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 the Serre-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 a multilinear map

:"f": "V""k" → "X "

such that whenever "v"1,...,"v""k" are linearly 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 unique linear map φ: Λ"k"("V") → "X" with "f" = φ o "w". This universal 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 the dual space of "V". In particular, the dimension of the space of anti-symmetric maps from "V""k" to "K" is the binomial 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 of tensor products 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:

:$omegawedgeeta=frac\left\{\left(k+m\right)!\right\}\left\{k!,m!\right\}\left\{ m Alt\right\}\left(omegaotimeseta\right)$

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

:$\left\{ m Alt\right\}\left(omega\right)\left(x_1,ldots,x_k\right)=frac\left\{1\right\}\left\{k!\right\}sum_\left\{sigmain S_k\right\}\left\{ m sgn\right\}\left(sigma\right),omega\left(x_\left\{sigma\left(1\right)\right\},ldots,x_\left\{sigma\left(k\right)\right\}\right)$

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:

:$omega wedge eta\left(x_1,ldots,x_\left\{k+m\right\}\right) = sum_\left\{sigma in Sh_\left\{k,m \left\{ m sgn\right\}\left(sigma\right),omega\left(x_\left\{sigma\left(1\right)\right\}, ldots, x_\left\{sigma\left(k\right)\right\}\right) eta\left(x_\left\{sigma\left(k+1\right)\right\}, ldots, x_\left\{sigma\left(k+m\right)\right\}\right),$

where here "Sh""k","m" ⊂ "S""k"+"m" is the subset of ("k,m") shuffles: permutations 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

:$omegawedgeeta=\left\{ m Alt\right\}\left(omegaotimeseta\right).$

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 a coalgebra.

The coproduct is a linear function Δ : Λ("V") → Λ("V") ⊗ Λ("V") given on decomposable elements by:$Delta\left(x_1wedgedotswedge x_k\right) = sum_\left\{p=0\right\}^k sum_\left\{sigmain Sh_\left\{p,k-p \left\{ m sgn\right\}\left(sigma\right) \left(x_\left\{sigma\left(1\right)\right\}wedgedotswedge x_\left\{sigma\left(p\right)\right\}\right)otimes \left(x_\left\{sigma\left(p+1\right)\right\}wedgedotswedge x_\left\{sigma\left(k\right)\right\}\right).$For example,

:$Delta\left(x_1\right) = 1 otimes x_1 + x_1 otimes 1,$

:$Delta\left(x_1 wedge x_2\right) = 1 otimes \left(x_1 wedge x_2\right) + x_1 otimes x_2 - x_2 otimes x_1 + \left(x_1 wedge x_2\right) otimes 1.$

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:

:

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 a bialgebra 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),

:$i_alpha:Lambda^k V ightarrowLambda^\left\{k-1\right\}V.$

This derivation is called the interior product with α, or sometimes the insertion operator, or contraction by α.

Suppose that w ∈ Λk"V". Then w is a multilinear mapping of "V"* to R, so it is defined by its values on the "k"-fold Cartesian product "V"*× "V"*× ... × "V"*. If "u"1, "u"2, ..., "u"k-1 are "k-1" elements of "V"*, then define

:

Additionally, let "i"α"f" = 0 whenever "f" is a pure scalar (i.e., belonging to Λ0"V").

Axiomatic characterization and properties

The interior product satisfies the following properties:

# For each "k" and each α ∈ V*,
#::$i_alpha:Lambda^kV ightarrow Lambda^\left\{k-1\right\}V.$
#:(By convention, Λ−1 = 0.)
# If "v" is an element of "V" ( = Λ1"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:
#::$i_alpha \left(awedge b\right) = \left(i_alpha a\right)wedge b + \left(-1\right)^\left\{deg a\right\}awedge \left(i_alpha b\right).$

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::* $i_alphacirc i_alpha = 0.$:*

Hodge duality

Suppose that "V" has finite dimension "n". Then the interior product induces a canonical isomorphism of vector spaces:$Lambda^k\left(V^*\right) otimes Lambda^n\left(V\right) o Lambda^\left\{n-k\right\}\left(V\right).$In the geometrical setting, a non-zero element of the top exterior power Λn(V) (which is a one-dimensional vector space) is sometimes called a volume form (or orientation form, although this term may sometimes lead to ambiguity.) Relative to a given volume form σ, the isomorphism is given explicitly by

:$alpha in Lambda^k\left(V^*\right) mapsto i_alphasigma in Lambda^\left\{n-k\right\}\left(V\right).$

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)

:$* : Lambda^k\left(V\right) ightarrow Lambda^\left\{n-k\right\}\left(V\right).$

The composite of * with itself maps Λk("V") → Λk("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 an orthonormal basis of "V". In this case,:$*circ * : Lambda^k\left(V\right) o Lambda^k\left(V\right) = \left(-1\right)^\left\{k\left(n-k\right) + q\right\}I$where "I" is the identity, and the inner product has metric 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

:$Lambda\left(f\right) : Lambda\left(V\right) ightarrow Lambda\left(W\right)$

such that

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

In particular, Λ("f") preserves homogeneous degree. The "k"-graded components of Λ("f") are given on decomposable elements by:$Lambda\left(f\right)\left(x_1wedge dots wedge x_k\right) = f\left(x_1\right)wedgedotswedge f\left(x_k\right).$

Let

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

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: the determinant of "f".

Exactness

If

:$0 ightarrow U ightarrow V ightarrow W ightarrow 0$

is a short exact sequence of vector spaces, then

:$0 o Lambda^1\left(U\right)wedgeLambda\left(V\right) o Lambda\left(V\right) ightarrow Lambda\left(W\right) ightarrow 0$

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:$0 o Lambda\left(U\right) oLambda\left(V\right).$ [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:

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

This is a graded isomorphism; i.e.,

:

Slightly more generally, if

:$0 ightarrow U ightarrow V ightarrow W ightarrow 0$

is a short exact sequence of vector spaces then $Lambda^k\left(V\right)$ has a filtration

:$0 = F^0 subseteq F^1 subseteq dotsb subseteq F^k subseteq F^\left\{k+1\right\} = Lambda^k\left(V\right)$

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

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

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

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

is exact. [Such a filtration also holds for vector bundles, 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 of antisymmetric tensors. Recall that the exterior algebra is the quotient of T("V") by the ideal "I" generated by "x" ⊗ "x".

Let Tr("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

:$ext\left\{Alt\right\}\left(v_1otimesdotsotimes v_r\right) = frac\left\{1\right\}\left\{r!\right\}sum_\left\{sigmainmathfrak\left\{S\right\}_r\right\} \left\{ m sgn\right\}\left(sigma\right) v_\left\{sigma\left(1\right)\right\}otimesdotsotimes v_\left\{sigma\left(r\right)\right\}$

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 $widehat\left\{otimes\right\}$ defined by

:$t widehat\left\{otimes\right\} s = ext\left\{Alt\right\}\left(totimes s\right).$

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

:$A\left(V\right)cong Lambda\left(V\right).$

Index notation

Suppose that "V" has finite dimension "n", and that a basis e1, ..., e"n" of "V" is given. then any alternating tensor "t" ∈ A"r"("V") ⊂ "T""r"("V") can be written in index notation as

:$t = t^\left\{i_1i_2dots i_r\right\}, \left\{mathbf e\right\}_\left\{i_1\right\}otimes \left\{mathbf e\right\}_\left\{i_2\right\}otimesdotsotimes \left\{mathbf e\right\}_\left\{i_r\right\}$

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

:$twidehat\left\{otimes\right\} s = frac\left\{1\right\}\left\{\left(r+p\right)!\right\}sum_\left\{sigmain \left\{mathfrak S\right\}_\left\{r+p ext\left\{sgn\right\}\left(sigma\right)t^\left\{i_\left\{sigma\left(1\right)\right\}dots i_\left\{sigma\left(r\right)s^\left\{i_\left\{sigma\left(r+1\right)\right\}dots i_\left\{sigma\left(r+p\right) \left\{mathbf e\right\}_\left\{i_1\right\}otimes \left\{mathbf e\right\}_\left\{i_2\right\}otimesdotsotimes \left\{mathbf e\right\}_\left\{i_\left\{r+p.$

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:

:$\left(twidehat\left\{otimes\right\} s\right)^\left\{i_1dots i_\left\{r+p = t^\left\{ \left[i_1dots i_r\right\}s^\left\{i_\left\{r+1\right\}dots i_\left\{r+p\right\}\right] \right\}.$

The interior product may also be described in index notation as follows. Let $t = t^\left\{i_0i_1dots i_\left\{r-1$ be an antisymmetric tensor of rank "r". Then, for α ∈ "V"*, "i"αt is an alternating tensor of rank "r"-1, given by

:$\left(i_alpha t\right)^\left\{i_1dots i_\left\{r-1=rsum_\left\{j=0\right\}^nalpha_j t^\left\{ji_1dots i_\left\{r-1.$

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 oriented parallelepiped with edges "u", "v", and "w".

Projective geometry

Decomposable "k"-vectors in Λ"k""V" correspond to weighted "k"-dimensional subspaces of "V". In particular, the Grassmannian of "k"-dimensional subspaces of "V", denoted "Gr"k("V"), can be naturally identified with an algebraic subvariety of the projective space Pk"V"). This is called the Plücker embedding.

Differential geometry

The exterior algebra has notable applications in differential geometry, where it is used to define differential forms. A differential form at a point of a differentiable manifold is an alternating multilinear form on the tangent space at the point. Equivalently, a differential form of degree "k" is a linear 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 a differential algebra. The exterior derivative commutes with pullback along smooth mappings between manifolds, and it is therefore a natural differential operator. The exterior algebra of differential forms, equipped with the exterior derivative, is a differential complex whose cohomology is called the de Rham cohomology of the underlying manifold and plays a vital role in the algebraic topology of differentiable manifolds.

Representation theory

In representation theory, the exterior algebra is one of the two fundamental Schur functors on the category of vector spaces, the other being the symmetric algebra. Together, these constructions are used to generate the irreducible representations of the general linear group.

Physics

The exterior algebra is an archetypal example of a superalgebra, which plays a fundamental role in physical theories pertaining to fermions and supersymmetry. For a physical discussion, see Grassmann number. For various other applications of related ideas to physics, see superspace and supergroup (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 a vector 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 multivectors. It was thus a "calculus", much like the propositional 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 by Giuseppe 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 (notably Henri Poincaré, Élie Cartan, and Gaston Darboux) who applied Grassmann's ideas to the calculus of differential forms.

A short while later, Alfred North Whitehead, borrowing from the ideas of Peano and Grassmann, introduced his universal algebra. This then paved the way for the 20th century developments of abstract 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 a quadratic 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 in partial 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]
*

*:: 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 and Stokes'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 in multivariate calculus introduces the exterior algebra of differential forms adroitly into the calculus sequence for colleges.

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