Free algebra
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In abstract algebra, a free algebra is the noncommutative analogue of a polynomial ring.
Let R be a ring. The free algebra on n indeterminates, X1, ..., Xn, is the ring spanned by all linear combinations of products of the variables. This ring is denoted R<X1, ..., Xn>. Unlike in a polynomial ring, the variables do not commute. For example X1X2 does not equal X2X1.
With the obvious scalar multiplication R<X1, ..., Xn> forms an algebra over R.
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.
More generally, one can construct the free algebra R<E> on any set E of 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.