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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*{dual vector space} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{homological_algebra}{}\paragraph*{{Homological algebra}}\label{homological_algebra} [[!include homological algebra - contents]] \hypertarget{duality}{}\paragraph*{{Duality}}\label{duality} [[!include duality - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{definitions}{Definitions}\dotfill \pageref*{definitions} \linebreak \noindent\hyperlink{transpose_maps}{Transpose maps}\dotfill \pageref*{transpose_maps} \linebreak \noindent\hyperlink{dual_bases}{Dual bases}\dotfill \pageref*{dual_bases} \linebreak \noindent\hyperlink{double_duals}{Double duals}\dotfill \pageref*{double_duals} \linebreak \noindent\hyperlink{dual_spaces_as_dual_objects}{Dual spaces as dual objects}\dotfill \pageref*{dual_spaces_as_dual_objects} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} A \emph{dual vector space} is a [[dual in a closed category]] of [[vector spaces]] (or similar [[algebraic structures]]). Of course, this is a very restricted notion of [[space]]; but for spaces in [[geometry]], one usually uses the [[duality between space and quantity]] and speaks of the [[spectrum]] (not `dual space') of an [[algebra]]. In [[homotopy theory]], there are also [[Spanier-Whitehead dual]]s; and there are other notions of dual space in [[physics]]. \hypertarget{definitions}{}\subsection*{{Definitions}}\label{definitions} Let $K$ be a [[field]] (or any [[commutative ring|commutative]] [[rig]]), and let $V$ be a [[vector space]] (or [[module]]) over $K$. \begin{defn} \label{}\hypertarget{}{} The \textbf{dual space} or \textbf{dual module} of $V$ is the vector space $V^*$ of [[linear functionals]] on $V$. That is, $V^*$ is the [[internal hom]] $[V,K]$ (thinking of $K$ as a vector space over itself: a [[line]]). \end{defn} More generally, let $K$ and $L$ be [[rings]] (or [[rigs]]) \emph{not} assumed commutative, and let $V$ be a $K$-$L$-[[bimodule]]. \begin{defn} \label{}\hypertarget{}{} The \textbf{left dual module} of $V$ is the right $K$-module $^*V$ of left $K$-module [[homomorphisms]] from $V$ to $K$. The \textbf{right dual module} of $V$ is the left $L$-module $V^*$ of right $L$-module homomorphisms from $V$ to $L$. \end{defn} Now let $V$ be a [[topological vector space]] over the [[ground field]] $K$. \begin{defn} \label{LinearDualOfATopologicalVectorSpace}\hypertarget{LinearDualOfATopologicalVectorSpace}{} \textbf{(linear dual of a topological vector space)} Let $V$ be a [[topological vector space]] over the [[ground field]] $K$. The \textbf{dual space} of $V$ is the topological vector space $V^*$ of [[continuous linear functionals]] on $V$, equipped with the [[weak-star topology|weak-* topology]] (meaning the [[initial topology]] generated by the elements of $V$, viewed as themselves linear functionals on $V^*$). \end{defn} In principle, there is no conflict among these definitions, the most general case (so far) being a topological bimodule over two topological rigs; the non-topological cases simply involve [[discrete spaces]]. In practice, however, some complications are possible: \begin{itemize}% \item If the rig $K$ is an [[associative algebra|algebra]] over another rig $L$, then any $K$-module $V$ is also an $L$-module, but the dual as a $K$-module is different from the dual as an $L$-module. So one may speak of the \textbf{$K$-dual} or the \textbf{dual over $K$}. \item A topological vector space $V$ has an [[underlying]] discrete vector space, and these have different duals. So one speaks of the \textbf{topological dual} and the \textbf{algebraic dual} (respectively). If $V$ is considered with several different topologies (say called `weak' and `strong'), then one may speak of the \textbf{weak dual} and the \textbf{strong dual} (etc). \end{itemize} Logically, these duals take place in different [[categories]], which are related by various [[functors]]; the objects whose duals are being taken should not be conflated. In practice, however, these objects are identified, so the duals must be distinguished. \hypertarget{transpose_maps}{}\subsection*{{Transpose maps}}\label{transpose_maps} The operation $V \mapsto V^*$ extends to a [[contravariant functor]]. \begin{defn} \label{}\hypertarget{}{} The [[dual linear map]] or \textbf{transpose map} of a [[linear map]] $A\colon V\to W$, is the linear map $A^* = A^T\colon W^*\to V^*$, given by \begin{displaymath} \langle{A^*(w), v}\rangle = \langle{w, A(v)}\rangle \end{displaymath} for all $w$ in $W^*$ and $v$ in $V$. \end{defn} \begin{remark} \label{}\hypertarget{}{} This functor is, of course, the [[representable functor]] represented by $K$ as a vector space over itself (a [[line]]). \end{remark} \begin{remark} \label{}\hypertarget{}{} This construction is the notion of [[dual morphism]] applied in the [[monoidal category]] [[Vect]] with its [[tensor product]] monoidal structure. \end{remark} \hypertarget{dual_bases}{}\subsection*{{Dual bases}}\label{dual_bases} If $B$ is any [[basis]] of $V$, then we can sometimes turn $B$ into a basis $B^*$ of the dual space $V^*$. We will begin with the definition of what \emph{might} be the dual basis, cautioning that it is not always a basis: \begin{defn} \label{}\hypertarget{}{} Treating the basis $B$ as a [[family]] $(b_i)$ with index set $I$, the \textbf{dual basis} $B^*$ is the family $(b^i)$ (with the same index set) such that \begin{displaymath} b^i(b_j) = \delta^i_j \end{displaymath} (the [[Kronecker delta]]). \end{defn} Since $B$ is a basis of $V$, this formula defines $b^i$ for each index $I$, so $B^*$ exists; but in general there is no reason why $B^*$ should be a basis of $V^*$. However, if $V$ has [[finite set|finite]] [[dimension]], then $B^*$ is a basis of $V^*$. If $V$ is a [[Hilbert space]], and $B$ is a basis of $V$ in the Hilbert space sense (i.e., $B$ is a linearly independent set whose span is topologically dense in $V$), then also $B^*$ is a basis of the dual Hilbert space $V^*$. This is related to but different from the sort of [[dual basis]] applicable generally to [[projective modules]]. \hypertarget{double_duals}{}\subsection*{{Double duals}}\label{double_duals} The \textbf{[[double dual]]} of $V$ is simply the dual of the dual of $V$. There is a [[natural transformation]] from $V$ to its double dual: \begin{displaymath} \hat{x}(\lambda) = \lambda(x) , \end{displaymath} for $x$ in $V$, $\lambda$ in $V^*$, and consequently $\hat{x}$ in $V^{**}$. The space $V$ is called \textbf{[[reflexive Banach space|reflexive]]} if this natural transformation is an [[isomorphism]]. The reflexive spaces include all finite-dimensional vector spaces or modules over [[fields]] or [[division rings]], as well as all [[Hilbert spaces]], the [[Lebesgue spaces]] $L^p$ over a [[localizable measure|localisable measure space]] for $1 \lt p \lt \infty$, and others. \hypertarget{dual_spaces_as_dual_objects}{}\subsection*{{Dual spaces as dual objects}}\label{dual_spaces_as_dual_objects} A dual vector space is a [[dual object]] in the [[monoidal category]] [[Vect]] equipped with its [[tensor product]] monoidal structure. In general, the duality between $V$ and $V^*$ does \emph{not} make $Vect$ into a [[monoidal category with duals]]. However, if we restrict to spaces of [[F-finite|finite]] [[dimension]], then we get a [[compact category]]; finite-dimensional [[Hilbert spaces]] form a $\dagger$-[[dagger-compact category|compact category]], which is very nice indeed. \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \begin{example} \label{DualityBetweenVectorsAndCovectors}\hypertarget{DualityBetweenVectorsAndCovectors}{} \textbf{(duality between [[vectors]] and [[covectors]])} \begin{itemize}% \item [[tensor|Tensors]] in finite-dimensional [[differential geometry]] make heavy use of the duality between [[tangent spaces]] and [[cotangent spaces]]. That [[infinite-dimensional manifold|infinite-dimensional differential geometry]] is much harder is largely because this duality is no longer perfect. \end{itemize} \end{example} \begin{example} \label{VonNeumannAlgebras}\hypertarget{VonNeumannAlgebras}{} \textbf{([[von Neumann algebras]])} A [[von Neumann algebra]] (abstractly) is precisely a $C^*$-[[C-star-algebra|algebra]] whose underlying [[Banach space]] is the dual space of some (other) Banach space. One may equivalently \emph{define} a von Neumann algebra as a Banach space together with a $C^*$-algebra structure on its dual space (except that the [[morphisms]] go the other way, so one is more directly defining a noncommutative [[measurable space]], along the lines of [[noncommutative geometry]]). \end{example} \begin{example} \label{SpacesOFDistributionsAsDualVectorSpaces}\hypertarget{SpacesOFDistributionsAsDualVectorSpaces}{} \textbf{(spaces of [[distributions]])} The standard [[topological space|topology]] on the spaces $\mathcal{D}'$ of [[distributions]] is the dual space topology according to def. \ref{LinearDualOfATopologicalVectorSpace}. (e.g. \hyperlink{Hoermander90}{H\"o{}rmander 90, p. 38}) \end{example} See \emph{[[Riesz representation theorem]]} for more examples from [[functional analysis]]. \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[dual space]] \item [[dual vector bundle]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} Discussion in the context of [[distributions]] includes \begin{itemize}% \item [[Lars Hörmander]], section 2.1 of \emph{The analysis of linear partial differential operators}, vol. I, Springer 1983, 1990 \end{itemize} [[!redirects dual vector space]] [[!redirects dual vector spaces]] [[!redirects dual module]] [[!redirects dual modules]] [[!redirects left dual module]] [[!redirects left dual modules]] [[!redirects right dual module]] [[!redirects right dual modules]] [[!redirects dual topological vector space]] [[!redirects dual topological vector spaces]] [[!redirects dual Banach space]] [[!redirects dual Banach spaces]] [[!redirects dual Hilbert space]] [[!redirects dual Hilbert spaces]] [[!redirects alebraic dual space]] [[!redirects alebraic dual spaces]] [[!redirects alebraic dual]] [[!redirects alebraic duals]] [[!redirects topological dual space]] [[!redirects topological dual spaces]] [[!redirects topological dual]] [[!redirects topological duals]] [[!redirects linear dual]] [[!redirects linear duals]] \end{document}