I have time series with 72 terms and frequency 1/12 (1 cycle/12 months) and it is explained here that the frequency bin for positive frequency is 72/12+1 (+1 is because of MATLAB's syntax). But on the other side there is formula where frequency bin is k=fs/N, where N is length of time series (72 in my case) and fs is frequency (1/12 in my case). When I use this formula I get 1/(72*12), which is different from the upper 72/12, what is my mistake here?