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In the book of A Course in Algebra by E. B. Vinberg, at page $298$, it is given that,

An element $z \in V \otimes W$, i.e in the tensor product of the space $V$ and $W$, is called decomposable if it decomposes as $$z = x \otimes y \quad x\in V, y \in W$$

However, since $z \in V \otimes W$, by definition it has to be written as $$z = x \otimes y.$$

I mean is there any change it cannot be written in this form ? So I didn't get it what is special about decomposability of $z$.

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    $\begingroup$ Some tensors cannot simply be written as tensor products. Take the linear combination of two decomposable tensors, for instance. $\endgroup$ – Duncan Aug 5 '17 at 15:41
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Let $V=W$ be two dimensional, with basis $\{e,f\}$. Consider $$e\otimes e+f\otimes f.$$

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