So this is actually a physics question, but i have a problem with just the math part of it, as i don't really get how to approximate this sum Problem 7.1 The question is: how one gets Z_rot=1+3e^(-h^2/IkT) ? Is there some kind of formula for approximation ? Cheers. 2. Relevant equations 3. The attempt at a solution I tried to perform operations and to simplify the expression but got to a dead end.