# Definition:Tensor Product of Abelian Groups/Family

## Definition

Let $I$ be an indexing set.

Let $\family {G_i}_{i \mathop \in I}$ be a family of abelian groups.

Let $G = \ds \prod_{i \mathop \in I} G_i$ be their direct product.

### Definition 1: by universal property

Their tensor product is an ordered pair:

$\struct {\ds \bigotimes_{i \mathop \in I} G_i, \theta}$

where:

$\ds \bigotimes_{i \mathop \in I} G_i$ is an abelian group
$\theta: G \to \ds \bigotimes_{i \mathop \in I} G_i$ is a multiadditive mapping such that, for every pair $\tuple {C, \omega}$ where:
$C$ is an abelian group
$\omega : G \to C$ is a multiadditive mapping
there exists a unique group homomorphism $g : \ds \bigotimes_{i \mathop \in I} G_i \to C$ such that $\omega = g \circ \theta$.
$\xymatrix{ G \ar[d]_\theta \ar[r]^\omega & C\\ \ds \bigotimes_{i \mathop \in I} G_i \[email protected]{.>}[ru]_g }$

### Definition 2: construction

Their tensor product is the ordered pair:

$\struct {\ds \bigotimes_{i \mathop \in I} G_i, \theta}$

where:

$\ds \bigotimes_{i \mathop \in I} G_i$ is the quotient of the free abelian group $\Z \sqbrk G$ on $G$, by the subgroup generated by the elements of the form $\tuple {x + y, \family {z_i}_{i \mathop \ne j} } - \tuple {x, \family {z_i}_{i \mathop \ne j} } - \tuple {y, \family {z_i}_{i \mathop \ne j} }$
for $j \in I$, $x, y \in G_j$, $\family {z_i}_{i \mathop \ne j} \in \ds \prod_{i \mathop \ne j} G_i$, where we denote $\tuple {x, \family {z_i}_{i \mathop \ne j} }$ for:
the family in $G$ whose $j$th term is $x$ and whose $i$th term is $z_i$, for $i \ne j$
its image under the canonical mapping $G \to \Z \sqbrk G$.
$\theta : G \to \ds \bigotimes_{i \mathop \in I} G_i$ is the composition of the canonical mapping $G \to \Z \sqbrk G$ with the quotient group epimorphism $\Z \sqbrk G \to \ds \bigotimes_{i \mathop \in I} G_i$:
$G \hookrightarrow \Z \sqbrk G \twoheadrightarrow \ds \bigotimes_{i \mathop \in I} G_i$

## Sources

• 1974: N. Bourbaki: Algebra I: Chapter $\text {II}$. Linear Algebra $\S 3$. Tensor Products. $9$. Tensor product of families of multimodules