Top: variation of the large separation as a function of frequency for the star KIC1872166. Error bars correspond to the 1σ uncertainties. The general trend is due to the second-order term (the curvature) and the modulation is due to the presence of acoustic glitches. The red dashed line is a linear fit over the oscillations to estimate the curvature parameter. The blue dashed line is the value of the mean curvature given by Mosser et al. (2013)Bottom: variation of the large separation δ_n as a function of frequency after suppressing the curvature term. The dotted-dashed line shows a fit obtained for this star using Eq. (8).