Tensors: How to construct multilinear maps

linear algebramultilinear-algebratensors

A tensor is defined as follows; A tensor of type (r, s) on V is a multilinear map T : $(V^*)^r \times V^s \to R$. The set of tensors of type (r, s) on V will be denoted by $T^r_s(V)$.

It takes r functionals(or covectors) and s vectors and maps them to R multilinearly. Now, how do I construct such an arbitrary map with the given vectors and functionals? When there are only functionals, I could have something like f(v)g(w)h(x). But when there is a number of vectors and functions given, how an arbitrary map can be constructed? For instance, if I am asked "give me an example of a function", I could give something like "f(x,y) = $x^2 + 2xy + y^2$". How can I construct such examples for a given Tensor type? Mostly I could see only metric tensor, cross product, moment of inertia, stress tensor being given as examples. But how do I go about constructing an example?

Best Answer

Let's start with the building blocks.

If you have a covector $\omega$, you can of course just apply it to a vector, $v\mapsto \omega(v)$.

If you have a vector $v$, you also have a canonical map to its double dual, $\omega\mapsto \omega(v)$.

So now that you know how to deal with single vectors and covectors, let's look at the tensor products.

As you already observed, you can construct the tensor product of covectors by just multiplying the individual values when applying to vectors. This also works analogously when vectors are involved. So for example, if you have the covectors $\alpha$ and $\beta$ and the vector $a$, you can generate the $(1,2)$ tensor $$(\omega,v,w)\mapsto \omega(a)\alpha(v)\beta(w)$$ However not all tensors are of that form; indeed, that's a very special tensor. But now we can use the fact that tensors of the same type can be added. And therefore given covectors $\alpha_k,\beta_k$ and vectors $a_k$ you can write a more general tensor as $$(\omega,v,w)\mapsto\sum_k \omega(a_k) \alpha_k(v) \beta_k(w)$$ And it turns out that this is already the most general form of an $(1,2)$ tensor.

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