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Universal property of the Clifford algebra

Statement

Let k,V,B,Q be as in Universal Clifford algebra of a quadratic space, and let A be a unital associative k-algebra. For every k-linear map f:V→A satisfying f(v)2=Q(v)1A for all v∈V, there is a unique unital k-algebra homomorphism fˉ:Cl(V,Q)→A such that fˉ∘iQ=f.

Facts & Assumptions

Given: The quadratic space (V,Q), a unital associative k-algebra A, and a k-linear map f:V→A satisfying the stated quadratic relations.

[F1]

Every linear map from V to a unital associative k-algebra extends uniquely to a unital algebra homomorphism from Tk(V) (Universal property of the tensor algebra).

[F2]

A homomorphism from Tk(V) that kills every Clifford relation factors uniquely through Cl(V,Q) (Factorization through the Clifford quotient).

[F3]

The canonical map is iQ=qQ∘j, and the quotient map qQ is surjective (Universal Clifford algebra of a quadratic space).

[F4]

A unital k-algebra is a unital ring with a central unital structure map from k (Algebras over a commutative ring, central structure maps, and algebra homomorphisms).

Proof

technique · direct
1.1givenF1F4construct

By [F4], A has the central unital k-algebra structure required by [F1], so f extends uniquely to a unital k-algebra homomorphism f~:Tk(V)→A with f~∘j=f.

2.1givenstep 1.1F2

For every v∈V, f~(j(v)2−Q(v)1)=f(v)2−Q(v)1A=0. Thus [F2] gives a unique unital homomorphism fˉ:Cl(V,Q)→A satisfying fˉ∘qQ=f~.

3.1givenF1F3step 2.1∎

Since iQ=qQ∘j, step 2.1 gives fˉ∘iQ=f~∘j=f. Let g:Cl(V,Q)→A be any other unital k-algebra homomorphism with g∘iQ=f. Then g∘qQ:Tk(V)→A and f~ are unital algebra extensions of the same map f along j, because (g∘qQ)∘j=g∘iQ=f=f~∘j. Uniqueness in [F1] gives g∘qQ=f~=fˉ∘qQ. Since qQ is surjective by [F3], g=fˉ.

Remarks

For a k-linear map f satisfying f(v)2=Q(v)1A, the universal property is the commutative triangle below. The maps iQ and f are k-linear; the dashed arrow is the unique unital k-algebra homomorphism fˉ with fˉ∘iQ=f. The triangle is viewed on underlying vector spaces, so it does not assign an algebra structure to V.

VCl(V;Q)AiQf¹f

The figure summarizes step 3.1; the proof above establishes both existence and uniqueness independently of the illustration.

Depends on

Dependency tree · 2 levels

Sources