Quadratic algebra

Short description: Algebraic structure in mathematics

In mathematics, a quadratic algebra is an algebra over a ring in which every element satisfies a quadratic equation. There are free and filtered quadratic algebras.

Free quadratic algebras

Given a commutative ring R, and the ring of polynomials R[X], a free quadratic algebra may be defined as quotient ring by a polynomial ideal: "An R-algebra of the form R[X]/(X² − a X − b) where X² − a X − b is a monic quadratic polynomial in R[X] and (X² − a X − b) Is the ideal it generates, is a free quadratic algebra over R."

Filtered quadratic algebras

A quadratic algebra may be a filtered algebra generated by degree one elements, with defining relations of degree 2. It was pointed out by Yuri Manin that such algebras play an important role in the theory of quantum groups. The most important class of graded quadratic algebras is Koszul algebras.

Graded quadratic algebras

A graded quadratic algebra A is determined by a vector space of generators V = A₁ and a subspace of homogeneous quadratic relations S ⊂ V ⊗ V. Thus

$A=T(V)/⟨S⟩$

and inherits its grading from the tensor algebra T(V).

If the subspace of relations is instead allowed to also contain inhomogeneous degree 2 elements, i.e. S ⊂ k ⊕ V ⊕ (V ⊗ V), this construction results in a filtered quadratic algebra.

A graded quadratic algebra A as above admits a quadratic dual: the quadratic algebra generated by V^* and with quadratic relations forming the orthogonal complement of S in V^* ⊗ V^*.

Examples

• The tensor algebra, symmetric algebra and exterior algebra of a finite-dimensional vector space are graded quadratic (in fact, Koszul) algebras.
• The universal enveloping algebra of a finite-dimensional Lie algebra is a filtered quadratic algebra.
• The Clifford algebra of a finite-dimensional vector space equipped with a quadratic form is a filtered quadratic algebra.
• The Weyl algebra of a finite-dimensional symplectic vector space is a filtered quadratic algebra.

Further reading

• Mazorchuk, Volodymyr; Ovsienko, Serge; Stroppel, Catharina (2009), "Quadratic duals, Koszul dual functors, and applications", Trans. Amer. Math. Soc. 361 (3): 1129–1172, doi:10.1090/S0002-9947-08-04539-X, https://www.ams.org/journals/tran/2009-361-03/S0002-9947-08-04539-X/home.html 
• Partha Serathi Chacraborty & Satyajit Guin (2015) Noncommuttive differential calculus on a quadratic algebra, Indian Journal of Pure and Applied Mathematics 46: 495 to 515.