n here is "generation". Start with one engineer, n=0. They refer 2, n=1. They each refer 2, referring a total of 4, n=2. Etc. (It works out such that the t

n here is "generation". Start with one engineer, n=0. They refer 2, n=1. They each refer 2, referring a total of 4, n=2. Etc. (It works out such that the total number of people follows the same curve. Think of the total number of engineers as (2^n)-1: following the same example above, one engineer is then n=1, three total is n=2, seven total is n=3.)