[Math] Tensor products

linear algebratensor-products

I'm trying to get my head round tensor products of vector spaces (I'm happy to see arguments in a more general setting, though).

I am concerned principally with two statements:

i) If $U,V,W$ are vector spaces then there is a one-to-one correspondence $\{ \mathrm{linear \ maps} \ V \otimes W \to U \} \longleftrightarrow \{ \mathrm{bilinear \ maps} \ V\times W \to U \} $.

ii) There is a natural (basis-independent) isomorphism $ (U \oplus V) \otimes W \to (U \otimes W) \oplus (V \otimes W)$

For the first of these statements, I can see map from left to right; any linear map $\phi : V \otimes W \to U$ gives rise to a bilinear map $ V \times W \to V \otimes W \to U$, where the first of these maps is the canonical map $p: (v,w) \mapsto v \otimes w$ and the second is $\phi$. I can't see, however, why any bilinear map $V \times W \to U$ necessarily factors into $\phi \circ p$ for some suitable linear map $\phi$.

I haven't got much experience with commutative diagrams. I think I've convinced myself that ii) is true with a commutative diagram, but I don't know if it's correct (and I also don't know how to LaTeX it easily…)

Any help would be appreciated. Thanks!

Best Answer

In your definition (you should really look up the universal property of the tensorproduct!) you can argue as follows:

If $b$ is a bilinear map $V\times W \rightarrow U$, you can simply define a linear map $$ v_i \otimes w_j \mapsto b(v_i,w_j) $$ since you know the $v_i\otimes w_j$ are a basis and a linear map can be defined by choosing arbitrary values for the elements of a basis.

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