Relating different ways of writing the outer product.

dual-spacesouter producttensor-productstensorsvector-spaces

I am having a hard time seeing how a a few concepts relate to each other. I an specifically coming at this from the perspective of tensors.

If I take two regular vectors from $\Bbb R^2$:

$$u,v\in \Bbb R^2$$

And perform the outer product on them.

$$u\otimes v=\begin{pmatrix}u_1v_1 & u_1v_2 \\ u_2v_1 & u_2v_2\end{pmatrix}$$

This is to my understanding in tensor language the tensor product of two vectors. However, it also appears that this can also be written as the matrix product

$$uv^T=\begin{pmatrix}u_1v_1 & u_1v_2 \\ u_2v_1 & u_2v_2\end{pmatrix}$$

I understand notationally how these produce the same result, my problem is that (seemingly hand-wavingly) taking the tranpose of a vector changes it to a covector, which I understand belongs in the dual space but which I also thought constituted a totally different object. Is the relationship between a vector in V and what I might consider the "same vector but transposed" in V* allowed to be defined in this way:

$$v\in V, w\in V^*$$

$$v^T=w$$

It seems strange to me that two completely different objects from the perspective of tensors (i.e. a (0,1) and (1,0) tensor) are allowed to be interchanged like this.

Disclaimer: I am studying physics not mathematics so please have mercy on my abilities.

Best Answer

It'll help to first discuss inner products. Do we denote them $\langle u,\,v\rangle$ or $\langle u|v\rangle$? It's an important notational distinction. Let's start with the first one, which just requires a sesquilinear positive-definite map from vector pairs to scalars. Once we have this in place, we can define $\langle u|$ as the linear map from $|v\rangle$ to $\langle u,\,v\rangle$, then write this map's evaluation $\langle u,\,v\rangle$ at $|v\rangle$ as $\langle u|v\rangle$. Doing this, we note the set of linear maps considered here is a vector space, and we call it the dual space of the original one. So now we identify each "vanilla" vector with a specific function, and that function gets called a covector.

Now we can turn to outer products: just as $\langle u|$ is a linear map, $|u\rangle\langle v|$ is a symbol for a sesquilinear map from a covector $\langle a|$ and vector $|b\rangle$ to the scalar $\langle a,\,u\rangle\cdot\langle b,\,v\rangle$, where $\cdot$ denotes multiplication of scalars.