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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{super vector space} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{superalgebra_and_supergeometry}{}\paragraph*{{Super-Algebra and Super-Geometry}}\label{superalgebra_and_supergeometry} [[!include supergeometry - contents]] \hypertarget{linear_algebra}{}\paragraph*{{Linear algebra}}\label{linear_algebra} [[!include homotopy - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{details}{Details}\dotfill \pageref*{details} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{StructuresInternalToSuperVectorSpaces}{Structures internal to super-vector spaces}\dotfill \pageref*{StructuresInternalToSuperVectorSpaces} \linebreak \noindent\hyperlink{delignes_theorem_on_tensor_categories}{Deligne's theorem on tensor categories}\dotfill \pageref*{delignes_theorem_on_tensor_categories} \linebreak \noindent\hyperlink{related_entries}{Related entries}\dotfill \pageref*{related_entries} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} A \textbf{super vector space} is an [[object]] in the non-trivial [[symmetric monoidal category]] structure on the [[monoidal category]] of $\mathbb{Z}/2$-[[graded vector spaces]]: as an object it is just a $\mathbb{Z}/2$-[[graded vector space]], but the [[braiding]] of the underlying [[tensor product of vector spaces]] is taken to be the non-trivial linear map which on elements of homogeneous degree is given by \begin{displaymath} \tau^{super} \;\colon\; v \otimes w \;\mapsto\; (-1)^{deg(v) deg(w)} \, w \otimes v \,. \end{displaymath} We make this precise as definition \ref{CategoryOfSuperVectorSpaces} below. Super vector spaces form the basis of [[superalgebra]] (over [[ground rings]] which are [[fields]]) in direct analogy of how ordinary vector spaces form the basis of ordinary [[algebra]]. For more on this see \hyperlink{StructuresInternalToSuperVectorSpaces}{below}, and for yet more see at \emph{[[geometry of physics -- superalgebra]]}. \hypertarget{details}{}\subsection*{{Details}}\label{details} \begin{defn} \label{VectorSpaces}\hypertarget{VectorSpaces}{} For $k$ a [[field]], we write $Vect_k$ for the [[category]] whose \begin{itemize}% \item [[object|objects]] are $k$-[[vector spaces]]; \item [[morphism|morphisms]] are $k$-[[linear functions]] between these. \end{itemize} When the [[ground field]] $k$ is understood or when its precise nature is irrelevant, we will often notationally suppress it and speak of just the category [[Vect]] of vector spaces. This is the category inside which [[linear algebra]] takes place. \end{defn} Of course the category [[Vect]] has some special properties. Not only are its objects ``linear spaces'', but the whole category inherits linear structure of sorts. This is traditionally captured by the following terminology for \textbf{[[additive and abelian categories]]}. Notice that there are several different but equivalent ways to state the following properties (discussed behind the relevant links). \begin{defn} \label{AdditiveAndAbelianCategories}\hypertarget{AdditiveAndAbelianCategories}{} Let $\mathcal{C}$ be a [[category]]. \begin{enumerate}% \item Say that $\mathcal{C}$ has \textbf{[[direct sums]]} if it has [[finite products]] and [[finite coproducts]] and if the canonical comparison morphism between these is an [[isomorphism]]. We write $V \oplus W$ for the direct sum of two objects of $\mathcal{C}$. \item Say that $\mathcal{C}$ is an \textbf{[[additive category]]} if it has [[direct sums]] and in addition it is [[Ab-enriched category|enriched in abelian groups]], meaning that every [[hom-set]] is equipped with the structure of an [[abelian group]] such that [[composition]] of morphisms is a [[bilinear map]]. \item Say that $\mathcal{C}$ is an \textbf{[[abelian category]]} if it is an [[additive category]] and has property that its [[monomorphisms]] are precisely the inclusions of [[kernels]] and its [[epimorphisms]] are precisely the projections onto [[cokernels]]. \end{enumerate} \end{defn} We also make the following definition of $k$-linear category, but notice that conventions differ as to which extra properties beyond [[Vect]]-[[enriched category|enrichment]] to require on a linear category: \begin{defn} \label{LinearCategory}\hypertarget{LinearCategory}{} For $k$ a [[field]] (or more generally just a [[commutative ring]]), call a [[category]] $\mathcal{C}$ a \textbf{$k$-[[linear category]]} if \begin{enumerate}% \item it is an [[abelian category]] (def. \ref{AdditiveAndAbelianCategories}); \item its [[hom-sets]] have the structure of $k$-[[vector spaces]] (generally $k$-[[modules]]) such that [[composition]] of morphisms in $\mathcal{C}$ is a [[bilinear map]] \end{enumerate} and the underlying additive [[abelian group]] structure of these [[hom-spaces]] is that of the underlying [[abelian category]]. In other words, a $k$-linear category is an [[abelian category]] with the additional structure of a [[Vect]]-[[enriched category]] (generally $k$[[Mod]]-enriched) such that the underlying [[Ab-enriched category|Ab-enrichment]] according to def. \ref{AdditiveAndAbelianCategories} is obtained from the $Vect$-enrichment under the [[forgetful functor]] $Vect \to Ab$. A [[functor]] between $k$-linear categories is called a \textbf{$k$-[[linear functor]]} if its component functions on [[hom-sets]] are [[linear maps]] with respect to the given $k$-linear structure, hence if it is a [[Vect]]-[[enriched functor]]. \end{defn} \begin{example} \label{}\hypertarget{}{} The category [[Vect]]${}_k$ of [[vector spaces]] (def. \ref{VectorSpaces}) is a $k$-[[linear category]] according to def. \ref{LinearCategory}. Here the abstract [[direct sum]] is the usual direct sum of [[vector spaces]], whence the name of the general concept. For $V,W$ two $k$-vector spaces, the vector space structure on the [[hom-set]] $Hom_{Vect}(V,W)$ of [[linear maps]] $\phi \colon V \to W$ is given by ``pointwise'' multiplication and addition of functions: \begin{displaymath} (c_1 \phi_1 + c_2 \phi_2) \;\colon\, v \;\mapsto\; c_1 \phi_1(v) + c_2 \phi_2(v) \end{displaymath} for all $c_1, c_2 \in k$ and $\phi_1, \phi_2 \in Hom_{Vect}(V,W)$. \end{example} Recall the basic construction of the [[tensor product of vector spaces]]: \begin{defn} \label{TensorProductOfVectorSpaces}\hypertarget{TensorProductOfVectorSpaces}{} Given two [[vector spaces]] over some [[field]] $k$, $V_1, V_2 \in Vect_k$, their [[tensor product of vector spaces]] is the vector space denoted \begin{displaymath} V_1 \otimes_k V_2 \;\in\; Vect \end{displaymath} whose elements are [[equivalence classes]] of [[tuples]] of elements $(v_1,v_2)$ with $v_i \in V_i$, for the [[equivalence relation]] given by \begin{displaymath} (c v_1 , v_2) \;\sim\; (v_1, c v_2) \end{displaymath} \begin{displaymath} (v_1 + v'_1 , v_2) \; \sim \; (v_1,v_2) + (v'_1, v_2) \end{displaymath} \begin{displaymath} (v_1 , v_2 + v'_2) \; \sim \; (v_1,v_2) + (v_1, v'_2) \end{displaymath} More abstractly this means that the [[tensor product of vector spaces]] is the vector space characterized by the fact that \begin{enumerate}% \item it receives a [[bilinear map]] \begin{displaymath} V_1 \times V_2 \longrightarrow V_1 \otimes V_2 \end{displaymath} (out of the [[Cartesian product]] of the underlying sets) \item any other [[bilinear map]] of the form \begin{displaymath} V_1 \times V_2 \longrightarrow V_3 \end{displaymath} factors through the above bilinear map via a unique [[linear map]] \begin{displaymath} \itexarray{ V_1 \times V_2 &\overset{bilinear}{\longrightarrow}& V_3 \\ \downarrow & \nearrow_{\mathrlap{\exists ! \, linear}} \\ V_1 \otimes_k V_2 } \end{displaymath} \end{enumerate} \end{defn} The existence of the [[tensor product of vector spaces]], def. \ref{TensorProductOfVectorSpaces}, equips the category [[Vect]] of vector spaces with extra structure, which is a ``[[categorification]]'' of the familiar structure of a [[semi-group]]. One also says ``[[monoid]]'' for [[semi-group]] and therefore [[categories]] equipped with a [[tensor product]] operation are also called \emph{[[monoidal categories]]}: \begin{defn} \label{MonoidalCategory}\hypertarget{MonoidalCategory}{} A \textbf{[[monoidal category]]} is a [[category]] $\mathcal{C}$ equipped with \begin{enumerate}% \item a [[functor]] \begin{displaymath} \otimes \;\colon\; \mathcal{C} \times \mathcal{C} \longrightarrow \mathcal{C} \end{displaymath} out of the [[product category]] of $\mathcal{C}$ with itself, called the \textbf{[[tensor product]]}, \item an object \begin{displaymath} 1 \in \mathcal{C} \end{displaymath} called the \textbf{[[unit object]]} or \textbf{[[tensor unit]]}, \item a [[natural isomorphism]] \begin{displaymath} a \;\colon\; ((-)\otimes (-)) \otimes (-) \overset{\simeq}{\longrightarrow} (-) \otimes ((-)\otimes(-)) \end{displaymath} called the \textbf{[[associator]]}, \item a [[natural isomorphism]] \begin{displaymath} \ell \;\colon\; (1 \otimes (-)) \overset{\simeq}{\longrightarrow} (-) \end{displaymath} called the \textbf{[[left unitor]]}, and a natural isomorphism \begin{displaymath} r \;\colon\; (-) \otimes 1 \overset{\simeq}{\longrightarrow} (-) \end{displaymath} called the \textbf{[[right unitor]]}, \end{enumerate} such that the following two kinds of [[commuting diagram|diagrams commute]], for all objects involved: \begin{enumerate}% \item \textbf{triangle identity}: \begin{displaymath} \itexarray{ & (x \otimes 1) \otimes y &\stackrel{a_{x,1,y}}{\longrightarrow} & x \otimes (1 \otimes y) \\ & {}_{\rho_x \otimes 1_y}\searrow && \swarrow_{1_x \otimes \lambda_y} & \\ && x \otimes y && } \end{displaymath} \item the \textbf{[[pentagon identity]]}: \begin{displaymath} \itexarray{ && (w \otimes x) \otimes (y \otimes z) \\ & {}^{\mathllap{\alpha_{w \otimes x, y, z}}}\nearrow && \searrow^{\mathrlap{\alpha_{w,x,y \otimes z}}} \\ ((w \otimes x ) \otimes y) \otimes z && && (w \otimes (x \otimes (y \otimes z))) \\ {}^{\mathllap{\alpha_{w,x,y}} \otimes id_z }\downarrow && && \uparrow^{\mathrlap{ id_w \otimes \alpha_{x,y,z} }} \\ (w \otimes (x \otimes y)) \otimes z && \underset{\alpha_{w,x \otimes y, z}}{\longrightarrow} && w \otimes ( (x \otimes y) \otimes z ) } \end{displaymath} \end{enumerate} \end{defn} As expected, we have the following basic example: \begin{example} \label{VectAsAMonoidalCategory}\hypertarget{VectAsAMonoidalCategory}{} For $k$ a [[field]], the category [[Vect]]${}_k$ of $k$-[[vector spaces]] becomes a [[monoidal category]] (def. \ref{MonoidalCategory}) as follows \begin{itemize}% \item the abstract [[tensor product]] is the [[tensor product of vector spaces]] $\otimes_k$ from def. \ref{TensorProductOfVectorSpaces}; \item the [[tensor unit]] is the [[field]] $k$ itself, regarded as a 1-dimensional vector space over itself; \item the [[associator]] is the map that on representing [[tuples]] acts as \begin{displaymath} \alpha_{V_{1}, V_2, V_3} \;\colon\; ((v_1, v_2), v_3) \mapsto (v_1, (v_2,v_3)) \end{displaymath} \item the left [[unitor]] is the map that on representing [[tuples]] is given by \begin{displaymath} \ell_{V} \colon (k,v) \mapsto k v \end{displaymath} and the right unitor is similarly given by \begin{displaymath} r_V \colon (v,k) \mapsto k v \,. \end{displaymath} \end{itemize} That this satisifes the [[pentagon identity]] (def. \ref{MonoidalCategory}) and the left and right unit identities is immediate on representing tuples. \end{example} But the point of the abstract definition of [[monoidal categories]] is that there are also more exotic examples. The followig one is just a minimal enrichment of example \ref{VectAsAMonoidalCategory}, and yet it will be important. \begin{example} \label{GradedVectorSpacesAsAMonoidaCategory}\hypertarget{GradedVectorSpacesAsAMonoidaCategory}{} Let $G$ be a [[group]] (or in fact just a [[monoid]]/[[semi-group]]). A \textbf{$G$-[[graded vector space]]} $V$ is a [[direct sum]] of vector spaces labeled by the elements in $G$: \begin{displaymath} V = \underset{g \in G}{\oplus} V_g \,. \end{displaymath} A [[homomorphism]] \begin{displaymath} \phi \;\colon\; V \longrightarrow W \end{displaymath} of $G$-graded vector spaces is a [[linear map]] that respects this direct sum structure, hence equivalently a [[direct sum]] of [[linear maps]] \begin{displaymath} \phi_g \;\colon\; V_g \longrightarrow W_g \end{displaymath} for all $g \in G$, such that \begin{displaymath} \phi = \underset{g \in G}{\oplus} \phi_g \,. \end{displaymath} This defines a [[category]], denoted $Vect^G$. Equip this category with a [[tensor product]] which on the underlying vector spaces is just the [[tensor product of vector spaces]] from def. \ref{TensorProductOfVectorSpaces}, equipped with the $G$-grading which is obtained by multiplying degree labels in $G$: \begin{displaymath} (V \otimes W)_g \;\coloneqq\; \underset{{g_1, g_2 \in G} \atop {g_1 g_2 = g}}{\oplus} V_{g_1} \otimes_k V_{g_2} \,. \end{displaymath} The [[tensor unit]] for the tensor product is the ground field $k$, regarded as being in the degree of the [[neutral element]] $e \in G$ \begin{displaymath} 1_g \;=\; \left\{ \itexarray{ k & | g = e \\ 0 & | otherwise } \right. \,. \end{displaymath} The [[associator]] and [[unitors]] are just those of the monoidal structure on plain vector spaces, from example \ref{VectAsAMonoidalCategory}. \end{example} One advantage of abstracting the concept of a [[monoidal category]] is that it allows to prove general statements uniformly for all kinds of tensor products, familiar ones and more exotic ones. The following lemma \ref{kel1} and remark \ref{CoherenceForMonoidalCategories} are two important such statements. \begin{lemma} \label{kel1}\hypertarget{kel1}{} \textbf{(\hyperlink{Kelly64}{Kelly 64})} Let $(\mathcal{C}, \otimes, 1)$ be a [[monoidal category]], def. \ref{MonoidalCategory}. Then the left and right [[unitors]] $\ell$ and $r$ satisfy the following conditions: \begin{enumerate}% \item $\ell_1 = r_1 \;\colon\; 1 \otimes 1 \overset{\simeq}{\longrightarrow} 1$; \item for all objects $x,y \in \mathcal{C}$ the following [[commuting diagram|diagrams commutes]]: \begin{displaymath} \itexarray{ (1 \otimes x) \otimes y & & \\ {}^\mathllap{\alpha_{1, x, y}} \downarrow & \searrow^\mathrlap{\ell_x \otimes id_y} & \\ 1 \otimes (x \otimes y) & \underset{\ell_{x \otimes y}}{\longrightarrow} & x \otimes y } \,; \end{displaymath} and \begin{displaymath} \itexarray{ x \otimes (y \otimes 1) & & \\ {}^\mathllap{\alpha^{-1}_{1, x, y}} \downarrow & \searrow^\mathrlap{id_x \otimes r_y} & \\ (x \otimes y) \otimes 1 & \underset{r_{x \otimes y}}{\longrightarrow} & x \otimes y } \,; \end{displaymath} \end{enumerate} \end{lemma} For \textbf{proof} see at \emph{[[monoidal category]]} \href{monoidal+category#kel1}{this lemma} and \href{monoidal+category#kel2}{this lemma}. \begin{remark} \label{CoherenceForMonoidalCategories}\hypertarget{CoherenceForMonoidalCategories}{} Just as for an [[associative algebra]] it is sufficient to demand $1 a = a$ and $a 1 = a$ and $(a b) c = a (b c)$ in order to have that expressions of arbitrary length may be re-bracketed at will, so there is a \emph{[[coherence theorem for monoidal categories]]} which states that all ways of freely composing the [[unitors]] and [[associators]] in a [[monoidal category]] (def. \ref{MonoidalCategory}) to go from one expression to another will coincide. Accordingly, much as one may drop the notation for the bracketing in an [[associative algebra]] altogether, so one may, with due care, reason about monoidal categories without always making all unitors and associators explicit. (Here the qualifier ``freely'' means informally that we must not use any non-formal identification between objects, and formally it means that the diagram in question must be in the image of a [[strong monoidal functor]] from a \emph{free} monoidal category. For example if in a particular monoidal category it so happens that the object $X \otimes (Y \otimes Z)$ is actually \emph{equal} to $(X \otimes Y)\otimes Z$, then the various ways of going from one expression to another using only associators \emph{and} this ``accidental'' equality no longer need to coincide.) \end{remark} The above discussion makes it clear that a [[monoidal category]] is like a [[monoid]]/[[semi-group]], but ``[[categorified]]''. Accordingly we may consider additional properties of [[monoids]]/[[semi-groups]] and correspondingly lift them to monoidal categories. A key such property is \emph{[[commutative ring|commutativity]]}. But while for a monoid commutativity is just an extra [[property]], for a [[monoidal category]] it involves choices of commutativity-[[isomorphisms]] and hence is [[stuff, structure and property|extra structure]]. We will see \hyperlink{SuperGroupsAsSuperHopfAlgebras}{below} that this is the very source of [[superalgebra]]. The [[categorification]] of ``commutativity'' comes in two stages: [[braiding]] and [[symmetric monoidal category|symmetric braiding]]. \begin{defn} \label{BraidedMonoidalCategory}\hypertarget{BraidedMonoidalCategory}{} A \textbf{[[braided monoidal category]]}, is a [[monoidal category]] $\mathcal{C}$ (def. \ref{MonoidalCategory}) equipped with a [[natural isomorphism]] \begin{displaymath} \tau_{x,y} \;\colon\; x \otimes y \to y \otimes x \end{displaymath} (for all [[objects]] $x,y in \mathcal{C}$) called the \textbf{[[braiding]]}, such that the following two kinds of [[commuting diagram|diagrams commute]] for all [[objects]] involved (``hexagon identities''): \begin{displaymath} \itexarray{ (x \otimes y) \otimes z &\stackrel{a_{x,y,z}}{\to}& x \otimes (y \otimes z) &\stackrel{\tau_{x,y \otimes z}}{\to}& (y \otimes z) \otimes x \\ \downarrow^{\tau_{x,y}\otimes Id} &&&& \downarrow^{a_{y,z,x}} \\ (y \otimes x) \otimes z &\stackrel{a_{y,x,z}}{\to}& y \otimes (x \otimes z) &\stackrel{Id \otimes \tau_{x,z}}{\to}& y \otimes (z \otimes x) } \end{displaymath} and \begin{displaymath} \itexarray{ x \otimes (y \otimes z) &\stackrel{a^{-1}_{x,y,z}}{\to}& (x \otimes y) \otimes z &\stackrel{\tau_{x \otimes y, z}}{\to}& z \otimes (x \otimes y) \\ \downarrow^{Id \otimes \tau_{y,z}} &&&& \downarrow^{a^{-1}_{z,x,y}} \\ x \otimes (z \otimes y) &\stackrel{a^{-1}_{x,z,y}}{\to}& (x \otimes z) \otimes y &\stackrel{\tau_{x,z} \otimes Id}{\to}& (z \otimes x) \otimes y } \,, \end{displaymath} where $a_{x,y,z} \colon (x \otimes y) \otimes z \to x \otimes (y \otimes z)$ denotes the components of the [[associator]] of $\mathcal{C}^\otimes$. \end{defn} \begin{defn} \label{SymmetricMonoidalCategory}\hypertarget{SymmetricMonoidalCategory}{} A \textbf{[[symmetric monoidal category]]} is a [[braided monoidal category]] (def. \ref{BraidedMonoidalCategory}) for which the [[braiding]] \begin{displaymath} \tau_{x,y} \colon x \otimes y \to y \otimes x \end{displaymath} satisfies the condition: \begin{displaymath} \tau_{y,x} \circ \tau_{x,y} = 1_{x \otimes y} \end{displaymath} for all objects $x, y$ \end{defn} \begin{remark} \label{SymmetricMonoidalCategoriesCoherenceTheorem}\hypertarget{SymmetricMonoidalCategoriesCoherenceTheorem}{} In analogy to the [[coherence theorem for monoidal categories]] (remark \ref{CoherenceForMonoidalCategories}) there is a [[coherence theorem for symmetric monoidal categories]] (def. \ref{SymmetricMonoidalCategory}), saying that every diagram built freely (see remark \ref{SymmetricMonoidalCategoriesCoherenceTheorem}) from [[associators]], [[unitors]] and [[braidings]] such that both sides of the diagram correspond to the same [[permutation]] of objects, coincide. \end{remark} Consider the simplest non-trivial special case of $G$-[[graded vector spaces]] from example \ref{GradedVectorSpacesAsAMonoidaCategory}, the case where $G = \mathbb{Z}/2$ is the [[cyclic group of order two]]. \begin{example} \label{Z2Zgradedvectorspaces}\hypertarget{Z2Zgradedvectorspaces}{} A \textbf{$\mathbb{Z}/2$-[[graded vector space]]} is a [[direct sum]] of two vector spaces \begin{displaymath} V = V_{even} \oplus V_{odd} \,, \end{displaymath} where we think of $V_{even}$ as the summand that is graded by the [[neutral element]] in $\mathbb{Z}/2$, and of $V_{odd}$ as being the summand that is graded by the single non-trivial element. A [[homomorphism]] of $\mathbb{Z}/2$-graded vector spaces \begin{displaymath} f \;\colon\; V_1 \longrightarrow V_2 \end{displaymath} is a [[linear map]] of the underlying vector spaces that respects the grading, hence equivalently a pair of linear maps \begin{displaymath} f_{even} \;\colon\; (V_1)_{even} \longrightarrow (V_1)_{even} \end{displaymath} \begin{displaymath} f_{odd} \;\colon\; (V_1)_{odd} \longrightarrow (V_1)_{odd} \end{displaymath} between then summands in even degree and in odd degree, respectively: \begin{displaymath} f = f_{even} \oplus f_{odd} \,. \end{displaymath} The [[tensor product]] of $\mathbb{Z}/2$-graded vector space is the [[tensor product of vector spaces]] of the underlying vector spaces, but with the grading obtained from multiplying the original gradings in $\mathbb{Z}/2$. Hence \begin{displaymath} (V_1 \otimes V_2)_{even} \;\coloneqq\; \left((V_1)_{even} \otimes (V_2)_{even}\right) \oplus \left((V_1)_{odd} \otimes (V_2)_{odd}\right) \end{displaymath} and \begin{displaymath} (V_1 \otimes V_2)_{odd} \;\coloneqq\; \left((V_1)_{even} \otimes (V_2)_{odd}\right) \oplus \left((V_1)_{odd} \otimes (V_2)_{even}\right) \,. \end{displaymath} As in example \ref{GradedVectorSpacesAsAMonoidaCategory}, this definition makes $\mathbb{Z}/2$ a [[monoidal category]] def. \ref{MonoidalCategory}. \end{example} \begin{prop} \label{TheTwoNontrivialBraidingsOnZ2GradedVectorSpaces}\hypertarget{TheTwoNontrivialBraidingsOnZ2GradedVectorSpaces}{} There are, up to [[braided monoidal functor|braided monoidal]] [[equivalence of categories]], precisely two choices for a [[symmetric monoidal category|symmetric]] [[braiding]] (def. \ref{SymmetricMonoidalCategory}) \begin{displaymath} V_1 \otimes V_2 \stackrel{\tau_{V_1,V_2}}{\longrightarrow} V_2 \otimes V_1 \end{displaymath} on the [[monoidal category]] $(Vect_k^{\mathbb{Z}/2}, \otimes_k)$ of $\mathbb{Z}/2$-[[graded vector spaces]] from def. \ref{Z2Zgradedvectorspaces}: \begin{enumerate}% \item the \textbf{trivial braiding} which is the [[natural transformation|natural]] [[linear map]] given on tuples $(v_1,v_2)$ representing an element in $V_1 \otimes V_2$ (according to def. \ref{TensorProductOfVectorSpaces}) by \begin{displaymath} \tau^{triv}_{V_1, V_2} \;\colon\; (v_1,v_2) \mapsto (v_2, v_1) \end{displaymath} \item the \textbf{super-braiding} which is the [[natural transformation|natural]] [[linear function]] given on tuples $(v_1,v_2)$ of \emph{homogeneous degree} (i.e. $v_i \in (V_i)_{\sigma_i} \hookrightarrow V_i$, for $\sigma_i \in \mathbb{Z}/2$) by \begin{displaymath} \tau^{super}_{V_1, V_2} \;\colon\; (v_1, v_2) \mapsto (-1)^{deg(v_1) deg(v_2)} \, (v_2,v_1) \,. \end{displaymath} \end{enumerate} \end{prop} \begin{proof} For $(\mathcal{C}, \otimes, 1)$ a [[monoidal category]], write \begin{displaymath} (Line(\mathcal{C}), \otimes, 1) \hookrightarrow (\mathcal{C}, \otimes, 1) \end{displaymath} for the [[full subcategory]] on those $L \in \mathcal{C}$ which are [[invertible objects]] under the [[tensor product]], i.e. such that there is an object $L^{-1} \in \mathcal{C}$ with $L \otimes L^{-1} \simeq 1$ and $L^{-1} \otimes L \simeq 1$. Since the [[tensor unit]] is clearly in $Line(L)$ (with $1^{-1} \simeq 1$) and since with $L_1, L_2 \in Line(\mathcal{C}) \hookrightarrow \mathcal{C}$ also $L_1 \otimes L_2 \in Line(\mathcal{C})$ (with $(L_1 \otimes L_2)^{-1} \simeq L_2^{-1} \otimes L_1^{-1}$) the [[monoidal category]] structure on $\mathcal{C}$ restricts to $Line(\mathcal{C})$. Accordingly any [[braiding]] on $(\mathcal{C}, \otimes,1)$ restricts to a braiding on $(Line(\mathcal{C}), \otimes, 1)$. Hence it is sufficient to show that there is an essentially unique non-trivial symmetric braiding on $(Line(\mathcal{C}), \otimes, 1)$, and that this is the restriction of a braiding on $(\mathcal{C}, \otimes, 1)$. Now $(Line(\mathcal{C}, \otimes , 1))$ is necessarily a [[groupoid]] (the ``[[Picard groupoid of a monoidal category|Picard groupoid]]'' of $\mathcal{C}$) and in fact is what is called a \emph{[[2-group]]}. As such we may regard it equivalently as a [[homotopy 1-type]] with group structure, and as such it it is equivalent to its [[delooping]] \begin{displaymath} B_\otimes Line(\mathcal{C}) \end{displaymath} regarded as a [[pointed homotopy type]]. (See at \emph{[[looping and delooping]]}). The [[Grothendieck group]] of $(\mathcal{C}, \otimes, 1)$ is \begin{displaymath} \pi_0(Line(\mathcal{C})) \simeq \pi_1(B Line(\mathcal{C})) \end{displaymath} the [[fundamental group]] of the delooping space. Now a symmetric braiding on $Line(\mathcal{C})$ is precisely the structure that makes it a [[symmetric 2-group]] which is equivalently the structure of a second [[delooping]] $B^2 Line(\mathcal{C})$ (for the braiding) and then a third delooping $B^3 Line(\mathcal{C})$ (for the symmetry), regarded as a [[pointed homotopy type]]. This way we have rephrased the question equivalently as a question about the possible [[k-invariants]] of spaces of this form. Now in the case at hand, $Line(Vect^{\mathbb{Z}/2})$ has precisely two [[isomorphism classes]] of objects, namely the [[ground field]] $k$ itself, regarded as being in even degree and regarded as being in odd degree. We write $k^{1\vert 0}$ and $k^{0 \vert 1}$ for these, respectively. By the rules of the tensor product of [[graded vector spaces]] we have \begin{displaymath} k^{1\vert 0} \otimes_k k^{1\vert 0} \simeq k^{1\vert 0} \end{displaymath} \begin{displaymath} k^{1\vert 0} \otimes_k k^{0\vert 1} \simeq k^{0\vert 1} \end{displaymath} and \begin{displaymath} k^{0 \vert 1} \otimes_k k^{0 \vert 1} \simeq k^{1 \vert 0} \,. \end{displaymath} In other words \begin{displaymath} \pi_0(Line(Vect^{\mathbb{Z}/2})) \simeq \mathbb{Z}/2 \,. \end{displaymath} Now under the above homotopical identification the non-trivial braiding is identified with the elements \begin{displaymath} 1 = k^{1 \vert 0} \simeq k^{0\vert 1} \otimes_k k^{0 \vert 1} \stackrel{\tau^{super}_{k^{0\vert 1}, k^{0 \vert 1}}}{\longrightarrow} k^{0\vert 1} \otimes_k k^{0\vert 1} \simeq k^{1 \vert 0} = 1 \end{displaymath} Due to the [[symmetric monoidal category|symmetry]] condition (def. \ref{SymmetricMonoidalCategory}) we have \begin{displaymath} (\tau^{super}_{k^{0\vert 1}, k^{0 \vert 1}})^2 = id \end{displaymath} which implies that \begin{displaymath} \tau^{super}_{k^{0\vert 1}, k^{0 \vert 1}} \in \{+ id, -id\} \,. \end{displaymath} Therefore for classifying just the symmetric braidings, it is sufficient to restrict the [[hom-spaces]] in $Line(Vect^{\mathbb{Z}/2})$ from being either $k$ or empty, to [[hom-sets]] being $\mathbb{Z}/2 = \{+1-1\} \hookrightarrow k$ or empty. Write $\widetilde{Line}(sVect)$ for the resulting [[2-group]]. In conclusion then the [[equivalence classes]] of possible [[k-invariants]] of $B^3 Line(sVect)$, hence the possible symmetric braiding on $Line(Vect^{\mathbb{Z}/2})$ are in the degree-4 [[ordinary cohomology]] of the [[Eilenberg-MacLane space]] $K(\mathbb{Z}/2,3)$ with [[coefficients]] in $\mathbb{Z}/2$. One finds (\ldots{}) \begin{displaymath} H^4(K(\mathbb{Z}/2, 3), \mathbb{Z}/2) \;\simeq\; \mathbb{Z}/2 \,. \end{displaymath} \end{proof} \begin{defn} \label{CategoryOfSuperVectorSpaces}\hypertarget{CategoryOfSuperVectorSpaces}{} The [[symmetric monoidal category]] (def. \ref{SymmetricMonoidalCategory}) \begin{itemize}% \item whose underlying [[monoidal category]] is that of $\mathbb{Z}/2$-[[graded vector spaces]] (example \ref{Z2Zgradedvectorspaces}); \item whose [[braiding]] (def. \ref{BraidedMonoidalCategory}) is the unique non-trivial symmetric grading $\tau^{super}$ from prop. \ref{TheTwoNontrivialBraidingsOnZ2GradedVectorSpaces} is called the \textbf{[[category of super vector spaces]]} \end{itemize} \begin{displaymath} sVect_k \;\coloneqq\; (Vect_k^{\mathbb{Z}/2}, \otimes = \otimes_k, 1 = k, \tau = \tau^{super} ) \,. \end{displaymath} \end{defn} \begin{remark} \label{}\hypertarget{}{} The non-full symmetric monoidal subcategory \begin{displaymath} (\widetilde{Line}(sVect), \otimes_k, k, \tau^{super}) \end{displaymath} of \begin{displaymath} (Line(sVect) , \otimes_k, k, \tau^{super}) \hookrightarrow (sVect, \otimes_k, k, \tau^{super}) \end{displaymath} (on the two objects $k^{1\vert 0}$ and $k^{0\vert 1}$ and with [[hom-sets]] restricted to $\{+1,-1\} \subset k$, as in the proof of prop. \ref{TheTwoNontrivialBraidingsOnZ2GradedVectorSpaces}) happens to be the [[truncated object of an (infinity,1)-category|1-truncation]] of the [[looping]] of the [[sphere spectrum]] $\mathbb{S}$, regarded as a group-like [[E-infinity space]] (``[[abelian infinity-group]]'') \begin{displaymath} (\widetilde{Line}(sVect), \otimes, k, \tau^{super}) \;\simeq\; \trunc_1 \Omega \mathbb{S} \,. \end{displaymath} It has been suggested (in \href{super+algebra#Kapranov15}{Kapranov 15}) that this and other phenomena are evidence that in the wider context of [[homotopy theory]]/[[stable homotopy theory]] super-grading (and hence [[superalgebra]]) is to be regarded as but a shadow of grading in [[higher algebra]] over the [[sphere spectrum]]. Notice that the [[sphere spectrum]] is just the analog of the group of [[integers]] in [[stable homotopy theory]]. \end{remark} \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{StructuresInternalToSuperVectorSpaces}{}\subsubsection*{{Structures internal to super-vector spaces}}\label{StructuresInternalToSuperVectorSpaces} By [[internalization|internalizing]] [[algebra]] and [[geometry]] in the category $sVect$ of super vector spaces, one obtains the corresponding [[superalgebra]] and [[supergeometry]]. For example \begin{itemize}% \item [[monoids]] in $sVect$ are [[super algebras]] and, more interestingly, [[commutative monoids]] in $sVect$ are [[supercommutative superalgebra]]; \item hence for example [[commutative Hopf algebras]] in $sVect$ are equivalently [[supercommutative Hopf algebras]], which are the [[formal duals]] of affine algebraic [[supergroups]]; \item [[Lie algebras]] in $sVect$ are [[super Lie algebras]]; \item [[manifolds]] modeled on objects of $sVect$ are [[supermanifolds]] \item etc. \end{itemize} By the above definition, any structure in $sVect$ works just like the corresponding structure in [[Vect]], but with a sign inserted whenever two odd-graded symbols are interchanged. For more on this see also at \emph{[[signs in supergeometry]]}. \hypertarget{delignes_theorem_on_tensor_categories}{}\subsubsection*{{Deligne's theorem on tensor categories}}\label{delignes_theorem_on_tensor_categories} [[Deligne's theorem on tensor categories]] says that all suitable [[tensor categories]] of subexponential growth have a [[fiber functor]] to $sVect$ and are equivalent to [[categories of representations]] of affine algebraic [[supergroups]]. \hypertarget{related_entries}{}\subsection*{{Related entries}}\label{related_entries} \begin{itemize}% \item [[supertrace]] \item [[superdeterminant]] \item [[super vector bundle]] \item [[chain complex of super vector spaces]] \item [[model structure on chain complexes of super vector spaces]] \item [[superalgebra]], [[supercommutative superalgebra]], [[differential graded-commutative superalgebra]], [[model structure on differential graded-commutative superalgebras]] \item [[geometry of physics -- supergeometry]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[Veeravalli Varadarajan]], section 3.1 of \emph{[[Supersymmetry for mathematicians]]: An introduction}, Courant lecture notes in mathematics, American Mathematical Society Providence, R.I 2004 \item [[Dennis Westra]], section 3 of \emph{Superrings and supergroups}, 2009 (\href{http://www.mat.univie.ac.at/~michor/westra_diss.pdf}{pdf}) \end{itemize} [[!redirects SVect]] [[!redirects sVect]] [[!redirects SuperVect]] [[!redirects super vector space]] [[!redirects super vector spaces]] [[!redirects supervector space]] [[!redirects supervector spaces]] [[!redirects super-vector space]] [[!redirects super-vector spaces]] [[!redirects category of super vector spaces]] [[!redirects category of super-vector spaces]] [[!redirects category of supervector spaces]] \end{document}