Does the projective tensor product obey a tensor-hom adjunction?

70 Views Asked by At

Let $X, Y, Z$ be three lctvs. Then https://ncatlab.org/nlab/show/inductive+tensor+product , the first theorem in section 3, tells us that $$\operatorname{Hom}(X\otimes_{\iota} Y, Z) = \operatorname{Hom}(X, L(Y, Z)_{\sigma}),$$

where $L(Y, Z)_{\sigma}$ is the space of continuous linear maps $Y\to Z$ given the topology of pointwise convergence.

Is there some topology on $L(Y,Z)$ I can use so that this holds with the inductive tensor product replaced by the projective tensor product?

1

There are 1 best solutions below

0
On BEST ANSWER

Assuming the homomorphisms of locally convex topological vector spaces are taken to be the continuous linear mappings, there are a few situations in which there is no locally convex topology on $L(Y,Z)$ compatible with the vector space structure for which $$\underset{\mathbf{lctvs}}{\operatorname{Hom}}(X\otimes_\pi Y,Z)=\underset{\mathbf{lctvs}}{\operatorname{Hom}}(X,L(Y,Z))$$ holds via the usual linear mapping $$\begin{array}{ccc}\underset{\mathbf{lctvs}}{\operatorname{Hom}}(X\otimes_\pi Y,Z)&\longrightarrow&\underset{\mathbf{Vect}}{\operatorname{Hom}}(X,L(Y,Z))\newline u&\longmapsto&x\mapsto u(x\otimes\cdot~)\end{array}.$$

For example, when $Y$ isn't seminormable and $Z$ is the scalar field, there is always a $X$ for which no good topology exists. More precisely, I show below the following result :

Let $Y$ be a locally convex space. Suppose that there exists a locally convex topology $\mathscr T$ on $Y'$ compatible with the vector space structure such that for every locally convex space $X$ and when $Y'$ is endowed with $\mathscr T$, the linear mapping $$\begin{array}{ccc}(X\otimes_\pi Y)'&\longrightarrow&\underset{\mathbf{Vect}}{\operatorname{Hom}}(X,Y')\newline u&\longmapsto&x\mapsto u(x\otimes\cdot~)\end{array}$$ realises a bijection between $(X\otimes_\pi Y)'$ and $L(X,Y')$. Then $Y$ is seminormable.


The plan is as follows : since the topology of $Y$ is the topology of uniform convergence on the convex balanced equicontinuous subsets of $Y'$, it suffices to show the existence of a convex balanced equicontinuous subsets of $Y'$ which absorbs every other convex balanced equicontinuous subset of $Y'$, as the gauge of the polar of such a set is a seminorm which defines the topology of $Y$.

For convenience, $Y'$ will from now on be endowed with $\mathscr T$ and for every convex balanced equicontinuous subset of $Y'$, I'll denote by $Y'_M$ the seminormable space obtained by endowing $\operatorname{span}(M)$ with the topology defined by the gauge of $M$. I'll denote by $X$ the locally convex space obtained by endowing $Y'$ with the finest locally convex topology such that for every convex balanced equicontinuous subset $M$ of $Y'$, the injection from $Y'_M$ to $X$ is continuous.


Notice that given a locally convex space $W$, a linear form $u$ on $W\otimes_\pi Y$ is continuous if and only if the bilinear form $(x,y)\mapsto u(x\otimes y)$ on $W\times Y$ is continuous, i.e. if and only if there is a neighbourhood $U$ of $0$ in $W$ and a neighbourhood $V$ of $0$ in $Y$ such that $|u(x\otimes y)|\leq 1$ for every $(x,y)\in U\times V$ or equivalently, if and only if there is a neighbourhood $U$ of $0$ in $W$ such that $\lbrace u(x\otimes\cdot~)~|~x\in U\rbrace$ is an equicontinuous subset of $Y'$. So by the assumptions on $\mathscr T$, a linear mapping $v$ from $W$ to $Y'$ is continuous if and only if there is a neighbourhood $U$ of $0$ in $W$ such that $v(U)$ is equicontinuous.

Then, for every convex balanced equicontinuous subset $M$ of $Y'$, the injection from $Y'_M$ to $Y'$ is continuous because $M$ is a neighbourhood of $0$ in $Y'_M$. It follows that the topology of $X$ is finer than that of $Y'$ and that the identity mapping from $X$ to $Y'$ is continuous. So there is a neighbourhood $U$ of $0$ in $X$ which is an equicontinuous subset of $Y'$.

Now, any convex balanced equicontinuous subset $M$ of $Y'$ is bounded in $Y'_M$ and so is bounded in $X$ because the injection from $Y'_M$ to $X$ is continuous. So $U$ absorbs every convex balanced equicontinuous subset of $Y'$.