Definition of the fluctuation function in the detrended fluctuation analysis and its variants Bastien Berthelot, Eric Grivel, Pierrick Legrand and Audrey Giremus Eur. Phys. J. B, 94 11 (2021) 225 Published online: 13 November 2021 DOI: 10.1140/epjb/s10051-021-00231-7