# A naive question about the double dual of a vector space

Let $K$ be a field. Are there non-scalar endomorphisms of the endofunctor $$V\mapsto V^{**}/V$$ of the category of $K$-vector spaces?

I asked a related question on Mathematics Stackexchange, but got no answer.

EDIT (Apr 15'14). Here is a closely related question which involves only basic linear algebra (and no category theory):

Let $K$ be a field and $V$ an infinite dimensional $K$-vector space. Is the $K$-algebra $$\operatorname{End}_{\operatorname{End}_K(V)}(V^{**}/V)$$ isomorphic to $K$?

EDIT (Apr 26'14). To avoid any misunderstanding, let me say explicitly that I'm unable to prove any of the following two statements:

(1) There is a pair $(K,V)$, where $K$ is a field and $V$ an infinite dimensional $K$-vector space, such that $$\operatorname{End}_{\operatorname{End}_K(V)}(V^{**}/V)\simeq K.$$

(2) There is a pair $(K,V)$, where $K$ is a field and $V$ an infinite dimensional $K$-vector space, such that $$\operatorname{End}_{\operatorname{End}_K(V)}(V^{**}/V)\not\simeq K.$$

EDIT (May 17'14). Here is a slight amplification of the above edit:

For any field $K$ and any infinite cardinal $\alpha$ put $$d(K,\alpha):=\dim_K\operatorname{End}_{\operatorname{End}_K(V)}(V^{**}/V),$$ where $V$ is an $\alpha$-dimensional $K$-vector space.

Let $\kappa$ be the cardinal of $K$. The Erdős-Kaplansky Theorem implies $$1\le d(K,\alpha)\le\kappa\wedge(\kappa\wedge(\kappa\wedge\alpha)),$$ where $\wedge$ denotes exponentiation.

There is no pair $(K,\alpha)$ for which I can prove that the first inequality is strict, and no pair $(K,\alpha)$ for which I can prove that the second inequality is strict.

EDIT (Jun 12'14). Here is a positive result. Unfortunately, it is very weak, and I hope users will be able to improve it.

Recall that $K$ is a field. For any vector space $V$ put $V':=V^{**}/V$. Let $V$ be an infinite dimensional vector space, and form the $K$-algebras $$A:=\operatorname{End}_KV,\quad B:=\operatorname{End}_KV',\quad C:=\operatorname{End}_AV',$$ so that $C$ is the commutant of $A$ in $B$. Recall that the dimension of $B$ is $$\dim B=\operatorname{card}K\wedge\Big(\operatorname{card}K\wedge\big(\operatorname{card}K\wedge\dim V\big)\Big),$$ where $\wedge$ denotes exponentiation. We claim

The codimension $c$ of $C$ in $B$ satisfies $\aleph_0\le c\le\dim B$.

To prove this, it suffices to show that, for each integer $n\ge2$, there is a subspace $W$ of $B$ satisfying $\dim W/(W\cap C)\ge n$.

Let $n$ be $\ge2$, let $V_1,\dots,V_n$ be subspaces of $V$ such that $V=V_1\oplus\cdots\oplus V_n$ and $\dim V_i=\dim V$ for all $i$, and let us define isomorphisms $\varphi_{ij}:V_j\to V_i$ for $1\le i,j\le n$ as follows.

Firstly $\varphi_{ii}$ is the identity of $V_i$. Secondly $\varphi_{i+1,i}:V_i\to V_{i+1}$, for $1\le i < n$, is any isomorphism. Thirdly $\varphi_{ij}$ for $j < i$ is the appropriate composition of the $\varphi_{k+1,k}$ previously defined. Fourthly we set $\varphi_{ij}:=(\varphi_{ji})^{-1}$ for $i < j$.

Define the morphism of $K$-algebras $\psi:M_n(K)\to B$ by $$\psi(a)(v)_i:=\sum_ja_{ij}\ \varphi'_{ij}(v_j)$$ for $v=v_1+\cdots+v_n\in V'$ with $v_i\in V'_i$. Then define $W$ as the image of $\psi$. It is easy to see that $\psi$ is injective and that $W\cap C$ is the image under $\psi$ of the scalar matrices, so that $\dim W/(W\cap C)=n^2-1\ge n$.

There is not a single case in which I'm able to improve the above inequalities $\aleph_0\le c\le\dim B$.

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Interesting question. I know there is a unique endomorphism of the functor $V \mapsto V^{**}$ that restricts to the identity on finite-dimensional vector spaces. (More generally, $(\ )^{**}$ is terminal among all endofunctors of Vect that restrict to the identity on FDVect.) This is similar in spirit to your question, and I was hoping to use it to answer your question, but I haven't managed. –  Tom Leinster Apr 2 '14 at 12:45
@TomLeinster - Dear Tom: Thanks a lot for your kind comment! Failing to prove your parenthetical remark, I did some googling, and found your paper Codensity and the ultrafilter monad, in particular Remark 7.6. I also found the links related to this paper on your webpage. Are these the good references for this fact? –  Pierre-Yves Gaillard Apr 2 '14 at 14:59
Yes, that's the paper I had in mind. I was really thinking of a simpler version of Proposition 5.4 (concerning just endofunctors, not monads). Double dualization is a codensity monad, i.e. a right Kan extension of a certain kind, and so has a certain universal property. This implies that it's terminal in the sense I mentioned. But there must surely be a more direct proof of this, and maybe it would be possible to adapt it to answer your actual question. –  Tom Leinster Apr 3 '14 at 3:24
So, you are saying you cannot do the computation when $K=\mathbb Q$ and $V = \mathbb Q[X]$ ...? –  Gerald Edgar May 17 '14 at 14:50
@GeraldEdgar - If $A$ is a $K$-algebra and $B$ is an $A$-module, then $\mathrm{End}_A(B)$ designates the $K$-vector space of all $A$-linear endomorphisms of $B$. –  Pierre-Yves Gaillard May 17 '14 at 22:01