# Characterisation of Linearly Independent Set through Free Module Indexed by Set

## Theorem

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

Let $S = \left\langle{m_i}\right\rangle_{i \mathop \in I}$ be a family of elements of $M$.

Let $\Psi : R^{\left({I}\right)} \to M$ be the module homomorphism given by Universal Property of Free Module on Set.

Then the following are equivalent:

- $S$ linearly independent
- $\Psi$ is injective.

## Proof

We have:

- $\Psi(\left\langle{r_i}\right\rangle_{i \mathop \in I}) = 0$

- $\displaystyle \sum_{i \mathop \in I} r_i m_i = 0$

Thus injectivity and linearly independent are equivalent.

$\blacksquare$