The frequency f3 is set to f1 + f2 because all other choices lead to vanishing cross-bispectra under stationarity assumption (Chella et al., 2016b; Shahbazi Avarvand et al., 2018). A pairwise bispectral analysis between two signals can be accomplished by setting two out of the three signal indices equal to each other, e.g., i = j which yields Bik (f1, f2) Biik (f1, f2).
