Hexagonal number

Template:Otheruses4 In mathematics, especially in the area of abstract algebra known as ring theory, a free algebra is the noncommutative analogue of a polynomial ring (which may be regarded as a free commutative algebra).

Definition

For R a commutative ring, the free (associative, unital) algebra on n indeterminates {X₁,...,X_n} is the free R-module with a basis consisting of all words over the alphabet {X₁,...,X_n} (including the empty word, which is the unity of the free algebra). This R-module becomes an R-algebra by defining a multiplication as follows: the product of two basis elements is the concatenation of the corresponding words:

$(X_{i_1}{X}_{i_2}\cdots X_{i_m})\cdot (X_{j_1}{X}_{j_2}\cdots X_{j_n})=X_{i_1}{X}_{i_2}\cdots X_{i_m}{X}_{j_1}{X}_{j_2}\cdots X_{j_n},$

and the product of two arbitrary elements is thus uniquely determined (because the multiplication in an R-algebra must be R-bilinear). This R-algebra is denoted R⟨X₁,...,X_n⟩. This construction can easily be generalized to an arbitrary set X of indeterminates.

In short, for an arbitrary set $X=\{X_i;i\in I\}$, the free (associative, unital) R-algebra on X is

$R⟨X⟩:=\underset{w\in X^{\ast }}{⨁}Rw$

with the R-bilinear multiplication that is concatenation on words, where X* denotes the free monoid on X (i.e. words on the letters X_i), $\oplus$ denotes the external direct sum, and Rw denotes the free R-module on 1 element, the word w.

For example, in R⟨X₁,X₂,X₃,X₄⟩, for scalars α,β,γ,δ ∈R, a concrete example of a product of two elements is $(\alpha X_1{X}_2^2+\beta X_2{X}_3)\cdot (\gamma X_2{X}_1+\delta X_1^4{X}_4)=\alpha \gamma X_1{X}_2^3{X}_1+\alpha \delta X_1{X}_2^2{X}_1^4{X}_4+\beta \gamma X_2{X}_3{X}_2{X}_1+\beta \delta X_2{X}_3{X}_1^4{X}_4$.

The non-commutative polynomial ring may be identified with the monoid ring over R of the free monoid of all finite words in the X_i.

Contrast with Polynomials

Since the words over the alphabet {X₁, ...,X_n} form a basis of R⟨X₁,...,X_n⟩, it is clear that any element of R⟨X₁, ...,X_n⟩ can be uniquely written in the form:

$\sum _{i_1,i_2,\cdots ,i_k\in \{1,2,\cdots ,n\}}{a}_{i_1,i_2,\cdots ,i_k}{X}_{i_1}{X}_{i_2}\cdots X_{i_k},$

where $a_{i_1,i_2,...,i_k}$ are elements of R and all but finitely many of these elements are zero. This explains why the elements of R⟨X₁,...,X_n⟩ are often denoted as "non-commutative polynomials" in the "variables" (or "indeterminates") X₁,...,X_n; the elements $a_{i_1,i_2,...,i_k}$ are said to be "coefficients" of these polynomials, and the R-algebra R⟨X₁,...,X_n⟩ is called the "non-commutative polynomial algebra over R in n indeterminates". Note that unlike in an actual polynomial ring, the variables do not commute. For example X₁X₂ does not equal X₂X₁.

More generally, one can construct the free algebra R⟨E⟩ on any set E of generators. Since rings may be regarded as Z-algebras, a free ring on E can be defined as the free algebra Z⟨E⟩.

Over a field, the free algebra on n indeterminates can be constructed as the tensor algebra on an n-dimensional vector space. For a more general coefficient ring, the same construction works if we take the free module on n generators.

The construction of the free algebra on E is functorial in nature and satisfies an appropriate universal property. The free algebra functor is left adjoint to the forgetful functor from the category of R-algebras to the category of sets.

Free algebras over division rings are free ideal rings.

See also

• Cofree coalgebra
• Tensor algebra
• Free object
• Noncommutative ring

References

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