From textbook, I know that:<br/>
If each $X_i$ is distributed as negative binomial $(r_i,p)$ then $\sum X_i$  is distributed as negative binomial $(\sum r_i ,p)$.<br/>
In R: `dnbinom(x, size=r, prob=p)`

However, I use a parametrization of negative binomial using mean and aggregation parameter [in R: `dnbinom(x, size, mu)` ]

What can I say from the sum of $X_i$ with such parametrization ?