Tensor algebra

In mathematics, the tensor algebra of a vector space V, denoted T or T, is the algebra of tensors on V with multiplication being the tensor product. It is the free algebra on V, in the sense of being left adjoint to the forgetful functor from algebras to vector spaces: it is the "most general" algebra containing V, in the sense of the corresponding universal property.
The tensor algebra is important because many other algebras arise as quotient algebras of T. These include the exterior algebra, the symmetric algebra, Clifford algebras, the Weyl algebra and universal enveloping algebras.
The tensor algebra also has two coalgebra structures; one simple one, which does not make it a bi-algebra, but does lead to the concept of a cofree coalgebra, and a more complicated one, which yields a bialgebra, and can be extended by giving an antipode to create a Hopf algebra structure.
Note: In this article, all algebras are assumed to be unital and associative. The unit is explicitly required to define the coproduct.

Construction

Let V be a vector space over a field K. For any nonnegative integer k, we define the kth tensor power of V to be the tensor product of V with itself k times:
That is, T^kV consists of all tensors on V of order k. By convention T⁰V is the ground field K.
We then construct T as the direct sum of T^kV for k = 0,1,2,…
The multiplication in T is determined by the canonical isomorphism
given by the tensor product, which is then extended by linearity to all of T. This multiplication rule implies that the tensor algebra T is naturally a graded algebra with T^kV serving as the grade-k subspace. This grading can be extended to a Z-grading by appending subspaces for negative integers k.
The construction generalizes in a straightforward manner to the tensor algebra of any module M over a commutative ring. If R is a non-commutative ring, one can still perform the construction for any R-''R bimodule M''.

Adjunction and universal property

The tensor algebra is also called the free algebra on the vector space, and is functorial; this means that the map extends to linear maps for forming a functor from the category of -vector spaces to the category of associative algebras. Similarly with other free constructions, the functor is left adjoint to the forgetful functor that sends each associative -algebra to its underlying vector space.
Explicitly, the tensor algebra satisfies the following universal property, which formally expresses the statement that it is the most general algebra containing V:
Here is the canonical inclusion of into. As for other universal properties, the tensor algebra can be defined as the unique algebra satisfying this property, but this definition requires to prove that an object satisfying this property exists.
The above universal property implies that is a functor from the category of vector spaces over, to the category of -algebras. This means that any linear map between -vector spaces and extends uniquely to a -algebra homomorphism from to.

Non-commutative polynomials

If V has finite dimension n, another way of looking at the tensor algebra is as the "algebra of polynomials over K in n non-commuting variables". If we take basis vectors for V, those become non-commuting variables in T, subject to no constraints beyond associativity, the distributive law and K-linearity.
Note that the algebra of polynomials on V is not, but rather : a linear function on V is an element of for example coordinates on a vector space are covectors, as they take in a vector and give out a scalar.

Quotients

Because of the generality of the tensor algebra, many other algebras of interest can be constructed by starting with the tensor algebra and then imposing certain relations on the generators, i.e. by constructing certain quotient algebras of T. Examples of this are the exterior algebra, the symmetric algebra, Clifford algebras, the Weyl algebra and universal enveloping algebras.

Coalgebra

The tensor algebra has two different coalgebra structures. One is compatible with the tensor product, and thus can be extended to a bialgebra, and can be further be extended with an antipode to a Hopf algebra structure. The other structure, although simpler, cannot be extended to a bialgebra. The first structure is developed immediately [|below]; the second structure is given in the section on the cofree coalgebra, further down.
The development provided below can be equally well applied to the exterior algebra, using the wedge symbol in place of the tensor symbol ; a sign must also be kept track of, when permuting elements of the exterior algebra. This correspondence also lasts through the definition of the bialgebra, and on to the definition of a Hopf algebra. That is, the exterior algebra can also be given a Hopf algebra structure.
Similarly, the symmetric algebra can also be given the structure of a Hopf algebra, in exactly the same fashion, by replacing everywhere the tensor product by the symmetrized tensor product, i.e. that product where
In each case, this is possible because the alternating product and the symmetric product obey the required consistency conditions for the definition of a bialgebra and Hopf algebra; this can be explicitly checked in the manner below. Whenever one has a product obeying these consistency conditions, the construction goes through; insofar as such a product gave rise to a quotient space, the quotient space inherits the Hopf algebra structure.
In the language of category theory, one says that there is a functor from the category of -vector spaces to the category of -associative algebras. But there is also a functor taking vector spaces to the category of exterior algebras, and a functor taking vector spaces to symmetric algebras. There is a natural map from to each of these. Verifying that quotienting preserves the Hopf algebra structure is the same as verifying that the maps are indeed natural.

Coproduct

The coalgebra is obtained by defining a coproduct or diagonal operator
Here, is used as a short-hand for to avoid an explosion of parentheses. The symbol is used to denote the "external" tensor product, needed for the definition of a coalgebra. It is being used to distinguish it from the "internal" tensor product, which is already being used to denote multiplication in the tensor algebra. In order to avoid confusion between these two symbols, most texts will replace by a plain dot, or even drop it altogether, with the understanding that it is implied from context. This then allows the symbol to be used in place of the symbol. This is not done below, and the two symbols are used independently and explicitly, so as to show the proper location of each. The result is a bit more verbose, but should be easier to comprehend.
The definition of the operator is most easily built up in stages, first by defining it for elements and then by homomorphically extending it to the whole algebra. A suitable choice for the coproduct is then
and
where is the unit of the field. By linearity, one obviously has
for all It is straightforward to verify that this definition satisfies the axioms of a coalgebra: that is, that
where is the identity map on. Indeed, one gets
and likewise for the other side. At this point, one could invoke a lemma, and say that extends trivially, by linearity, to all of, because is a free object and is a generator of the free algebra, and is a homomorphism. However, it is insightful to provide explicit expressions. So, for, one has the homomorphism
Expanding, one has
In the above expansion, there is no need to ever write as this is just plain-old scalar multiplication in the algebra; that is, one trivially has that
The extension above preserves the algebra grading. That is,
Continuing in this fashion, one can obtain an explicit expression for the coproduct acting on a homogenous element of order m:
where the symbol, which should appear as ш, the sha, denotes the shuffle product. This is expressed in the second summation, which is taken over all -shuffles. The shuffle is
By convention, one takes that Sh and Sh equals. It is also convenient to take the pure tensor products
and
to equal 1 for p = 0 and p = m, respectively. The shuffle follows directly from the first axiom of a co-algebra: the relative order of the elements is preserved in the riffle shuffle: the riffle shuffle merely splits the ordered sequence into two ordered sequences, one on the left, and one on the right.
Equivalently,
where the products are in, and where the sum is over all subsets of.
As before, the algebra grading is preserved:

Counit

The counit is given by the projection of the field component out from the algebra. This can be written as for and for. By homomorphism under the tensor product, this extends to
for all
It is a straightforward matter to verify that this counit satisfies the needed axiom for the coalgebra:
Working this explicitly, one has
where, for the last step, one has made use of the isomorphism, as is appropriate for the defining axiom of the counit.

Bialgebra

A bialgebra defines both multiplication, and comultiplication, and requires them to be compatible.

Multiplication

Multiplication is given by an operator
which, in this case, was already given as the "internal" tensor product. That is,
That is, The above should make it clear why the symbol needs to be used: the was actually one and the same thing as ; and notational sloppiness here would lead to utter chaos. To strengthen this: the tensor product of the tensor algebra corresponds to the multiplication used in the definition of an algebra, whereas the tensor product is the one required in the definition of comultiplication in a coalgebra. These two tensor products are not the same thing!