Anyone help me this question
Show that a R-module $M$ is a module flat if only if, such if exist R - dependence linear $\displaystyle \sum a_i m_i =0$ of elements of $M$ , will be exists $a_ij \in R$ and $m_{j}^{'} \in M$, such $\displaystyle \sum a_{ij}a_i = 0 \ \forall j$ and $m_i = \displaystyle \sum _{j} a_ij m_{j}^{'}$.
Thanks in advance