Exterior powers in tensor categories - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-20T02:42:16Z http://mathoverflow.net/feeds/question/70607 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/70607/exterior-powers-in-tensor-categories Exterior powers in tensor categories Martin Brandenburg 2011-07-18T10:02:39Z 2013-03-21T00:21:08Z <p>Let $\mathcal{C}$ be a cocomplete $R$-linear tensor category. Many notions of commutative algebra may be internalized to $\mathcal{C}$. For example an algebra is an object $A$ in $\mathcal{C}$ together with morphisms $e : 1 \to A$ (unit) and $m: A \otimes A \to A$ (multiplication) satisfying the usual laws. The $n$-th symmetric power $\text{Sym}^n(X)$ of an object $X$ is the quotient of $X^{\otimes n}$ by $x_1 \otimes ... \otimes x_n = x_{\sigma(1)} \otimes ... \otimes x_{\sigma(n)}$, so formally it is defined as a coequalizer of the $n!$ symmetries $X^{\otimes n} \to X^{\otimes n}$. Then $\text{Sym}(X) = \bigoplus_{n\geq 0} \text{Sym}^n(X)$ is an algebra object and $\text{Sym}$ is in fact left adjoint to the forgetful functor $\mathsf{CAlg}(\mathcal{C}) \to \mathsf{CAlg}(\mathcal{C})$.</p> <p>But now what about the <strong>exterior power</strong> $\Lambda^n(X)$? It is clear how to define $X^{\otimes n}$ modulo $x_1 \otimes ... \otimes x_n = \text{sgn}(\sigma) \cdot x_{\sigma(1)} \otimes ... \otimes x_{\sigma(n)}$ in this context, which one might call the <em>anti-symmetric power</em> $\mathrm{ASym}^n(X)$. The correct definition of the <em>exterior power</em> also has to mod out $... \otimes a \otimes ... \otimes a \otimes ... = 0$. But I have no idea how to internalize this to $\mathcal{C}$, even for $n=2$. The reason is that there is no morphism $X \to X \otimes X$ which acts like $a \mapsto a \otimes a$. Another idea would be to define $\Lambda(X)$ as a graded-commutative algebra object with the usual universal property, classifying morphisms $f$ on $X$ which satisfy something like $f(x)^2=0$, but again it is unclear how to formulate this in $\mathcal{C}$.</p> <p>If this is not possible at all, which additional structure on $\mathcal{C}$ do we need in order to define exterior powers within them? Is this some categorified $\lambda$-ring structure? This structure should be there in the case of usual module categories (over rings or even ringed spaces). Of course there is no problem when $2 \in R^*$, because then the exterior power equals the anti-symmetric power. The question was also discussed in a <a href="http://concretenonsense.wordpress.com/2009/07/23/lambda-rings/" rel="nofollow">blog</a> post.</p> <p>Here is a more specific (and a bit stronger) formulation: Is there some $R[\Sigma_n]$-module $T$, such that for every $R$-module $X$, we have that $T \otimes_{R[\Sigma_n]} X^{\otimes n} \cong \Lambda^n X := X^{\otimes n}/(... \otimes x ... \otimes x ...)$? </p> <p>Concerning the "hidden extra structure" in the case of modules: Let the base ring be $\mathbb{Z}$, or more generally a ring $R$ in which $r^2 - r \in 2R$ for all $r \in R$; this includes boolean rings such as $\mathbb{F}_2$ and also $\mathbb{Z}/n$. If $X$ is an $R$-module, then there is a well-defined(!) homomorphism $X^{\otimes~ n-1} \to \text{ASym}^n(X), x_1 \otimes ... \otimes x_n \mapsto x_1 \wedge x_1 \wedge ... \wedge x_n$, and its cokernel is $\Lambda^n(X)$.</p> http://mathoverflow.net/questions/70607/exterior-powers-in-tensor-categories/70728#70728 Answer by anon for Exterior powers in tensor categories anon 2011-07-19T12:04:37Z 2011-07-19T12:04:37Z <p>Deligne (Categories Tannakiennes, 1990, p165) defines it to be the image of the antisymmetrisation $a=\sum(-1)^{\epsilon(\sigma)}\sigma\colon X^{\otimes n}\rightarrow X^{\otimes n}$.</p>