The tensor product of an -module and an -module is an -module generated by pure tensors , subject to additivity in both variables and the balancing relation
A map is balanced when it is additive in each variable and satisfies .
Every balanced map factors uniquely through an -module homomorphism satisfying .
For modules over a commutative ring, there is a natural isomorphism
given by .
Tensor product need not commute with an infinite direct product. For example,
The element has infinite order in the product, so it survives rational localization.

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In algebra, the tensor product is a way to construct a new module from two given modules, effectively allowing us to "multiply" the modules together. It is particularly useful in the context of linear algebra, representation theory, and algebraic topology. ### Definition Let \( R \) be a ring, and let \( M \) and \( N \) be two \( R \)-modules.