Quadratic algebra

In mathematics, a quadratic algebra is 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.
Definition.
A graded quadratic algebra "A" is determined by a vector space of generators "V" = "A"1 and a subspace of homogeneous quadratic relations "S" ⊂ "V" ⊗ "V" . Thus
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"*.