From textbook, I know that : If each Xi is distributed as negative binomial (r_i,p) then ∑ X_i is distributed as negative binomial (∑ r_i ,p). 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 Xi with such parametrization ?

Thanks Marc