Finding Big O of 1/n + 1/n+2 ... 1/n'
Selah, instead of asking for more references and definitions, you can just use the fact that for the n values n ≤ k &lt; 2n, we have $1/{2n} ≤ 1/k ≤ 1/n$. So the sum of $1/k$ for n ≤ k &lt; 2n is between 1/2 and 1. 

Not nearly as nice as Yuval's result, but it should be more than good enough to solve your problem.