Intermittency is a property observed in turbulence and, in some sense, it represents how much real turbulence diverges from Kolmogorov's K41 theory. It is usually measured using the so-called flatness, based on structure functions in the physical space but also on Fourier high-pass filters. In experiments, the two approaches often give different measurements. After presenting these concepts, I will talk about the similarities and differences between these two flatnesses from an analytical point of view. To do so, I will compare them using Riemann's non-differentiable function, which has been related to turbulence, the multifractal formalism and the motion of vortex filaments before. This work follows the trend of "mathematizing" turbulence, and is in collaboration with Alexandre Boritchev and Victor Vilaça da Rocha.