The textbook I'm currently reading (Engineering Dynamics by Paley and Kasdin) states the internal work done on a system of $N$ particles (that is, the work done by all the internal forces in the system) is as follows:
$$\sum_{i=1}^N\sum_{j=1}^N\int_{C_i}\vec {F_{i, j}}\cdot d\vec{r_{i/o}}$$
where $C_i$ is the path that particle $i$ traverses and $\vec{F_{i,j}}$ is the internal force exerted by particle $j$ on particle $i$.
Now, it also claims that the expression above is equivalent to the following expression:
$$\frac{1}{2}\sum_{i=1}^N\sum_{j=1}^N\int_{C_i}\vec {F_{i, j}}\cdot d\vec{r_{i/j}}$$
My question is, how did the author get from the first expression to the other? The textbook skips the math but I still wanted to know how you get this result.
I'm pretty sure it uses the fact that
$$\sum_{i=1}^N\sum_{j=1}^N\int_{C_i}\vec {F_{i, j}}\cdot d\vec{r_{i/o}}=\sum_{i=1}^N\sum_{j=1}^N\int_{C_i}\vec {F_{j, i}}\cdot d\vec{r_{j/o}}$$
but I am not sure how to proceed from here.