Friday, 14 August 2015

ct.category theory - Clifford algebra as an adjunction?

Background



For definiteness (even though this is a categorical question!) let's agree that a vector space is a finite-dimensional real vector space and that an associative algebra is a finite-dimensional real unital associative algebra.



Let $V$ be a vector space with a nondegenerate symmetric bilinear form $B$ and let $Q(x) = B(x,x)$ be the associated quadratic form. Let's call the pair $(V,Q)$ a quadratic vector space.



Let $A$ be an associative algebra and let's say that a linear map $phi:V to A$ is Clifford if
$$phi(x)^2 = - Q(x) 1_A,$$
where $1_A$ is the unit in $A$.



One way to define the Clifford algebra associated to $(V,Q)$ is to say that it is universal for Clifford maps from $(V,Q)$. Categorically, one defines a category whose objects are pairs $(phi,A)$ consisting of an associative algebra $A$ and a Clifford map $phi: V to A$ and whose arrows
$$h:(phi,A)to (phi',A')$$
are morphisms $h: A to A'$ of associative algebras such that the obvious triangle commutes:
$$h circ phi = phi'.$$
Then the Clifford algebra of $(V,Q)$ is the universal initial object in this category. In other words, it is a pair $(i,Cl(V,Q))$ where $Cl(V,Q)$ is an associative algebra and $i:V to Cl(V,Q)$ is a Clifford map, such that for every Clifford map $phi:V to A$, there is a unique morphism
$$Phi: Cl(V,Q) to A$$
extending $phi$; that is, such that $Phi circ i = phi$.



(This is the usual definition one can find, say, in the nLab.)



Question



I would like to view the construction of the Clifford algebra as a functor from the category of quadratic vector spaces to the category of associative algebras. The universal property says that if $(V,Q)$ is a quadratic vector space and $A$ is an associative algebra, then there is a bijection of hom-sets



$$mathrm{hom}_{mathbf{Assoc}}(Cl(V,Q), A) cong mathrm{cl-hom}(V,A)$$



where the left-hand side are the associative algebra morphisms and the right-hand side are the Clifford morphisms.



My question is whether I can view $Cl$ as an adjoint functor in some way. In other words, is there some category $mathbf{C}$ such that the right-side is
$$mathrm{hom}_{mathbf{C}}((V,Q), F(A))$$
for some functor $F$ from associative algebras to $mathbf{C}$. Naively I'd say $mathbf{C}$ ought to be the category of quadratic vector spaces, but I cannot think of a suitable $F$.



I apologise if this question is a little vague. I'm not a very categorical person, but I'm preparing some notes for a graduate course on spin geometry next semester and the question arose in my mind.

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