Abstract Algebra – Tensors and Multilinear Maps Isomorphism

abstract-algebratensor-productstensors

Prove that there is a natural isomorphism between the space $T^{(k+1,l)}(V)$ of $(k+1,l)$-tensors over $V$ and the space of multilinear maps
$$
\underbrace{V^\ast \times \dots \times V^\ast}_{k \text{ factors }} \times \underbrace{V \times \dots \times V}_{l \text{ factors }} \to V.
$$
Here $V$ is a finite-dimensional vector space.

For any multilinear map $f : V^\ast \times \dots \times V^\ast \times V \times \dots \times V \to V$ the universal property of tensor products implies that there exists a linear $\tilde{f}: V^\ast \otimes \dots \otimes V^\ast \otimes V \otimes \dots \otimes V \to V$ that agrees with $f$ in the sense that $\tilde{f}(v^*_1 \otimes \dots \otimes v^*_k \otimes v_1\otimes\dots \otimes v_l) = f(v^*_1,\dots,v_k^*,v_1,\dots,v_l)$. Since $\tilde{f} \in \operatorname{Hom}(V^\ast \otimes \dots \otimes V^\ast \otimes V \otimes \dots \otimes V, V)$ and this space is isomorphic to $k+1$ copies of $V$ tensored with $l$ copies of $V^\ast$ the proof should follow.

Is this are there is to it or am I missing something obvious?

Best Answer

I think it might be less confusing to explain it this way:

First, given vector spaces $W_1, \dots, W_m$ and another vector space $V$, the universal property of the tensor product gives a natural isomorphism between the space of multilinear maps of the form $$ f: W_1\times\cdots\times W_m \rightarrow V $$ and the space of linear maps of the form $$ \tilde{f}: W_1\otimes \cdots \otimes W_m \rightarrow V. $$

Second, given vector spaces $T, V$, there is a natural isomorphism between the space of linear maps of the form $$ L: T \rightarrow V $$ and the space $T^*\otimes V$.

Third, There is a natural isomorphism $$ (W_1\otimes\cdots \otimes W_m)^* \simeq W_1^*\otimes \cdots\otimes W_m^*. $$

You can prove all of this yourself easily.

The case you're looking at is when $m = k+l$, $W_1=\cdots=W_k= V^*$, $W_{k+1}=\cdots=W_{k+l}=V$, and $$ T = W_1\otimes\cdots\otimes W_m = V^*\otimes\cdots V^*\otimes V\cdots\otimes V. $$

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