# Generated Submodule is Linear Combinations

## Theorem

Let $G$ be a unitary $R$-module.

Let $S \subseteq G$.

Then the submodule $H$ generated by $S$ is the set of all linear combinations of $S$.

## Proof

First the extreme case:

The smallest submodule of $G$ containing $\varnothing$ is $\left\{{e_G}\right\}$.

By definition, $\left\{{e_G}\right\}$ is the set of all linear combinations of $\varnothing$.

Now the general case:

Let $\varnothing \subset S \subseteq G$.

Let $L$ be the set of all linear combinations of $S$.

Since $G$ is a unitary $R$-module, every element $x \in S$ is the linear combination $1_R x$, so $S \subseteq L$.

But $L$ is closed for addition and scalar multiplication, so is a submodule.

Thus $H \subseteq L$.

But as every linear combination of $S$ clearly belongs to any submodule of $G$ which contains $S$, we also have $L \subseteq H$.

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): $\S 27$: Theorem $27.3$