The general model of variability that we fit to time series of Gaia observations is given by:

$$y=\sum _{n=1}^{{N}_{f}}\sum _{k=1}^{{N}_{h}(n)}{A}_{n,k}\mathrm{cos}(2\pi k{f}_{n}t+{\psi}_{n,k})+\sum _{i=0}^{{N}_{p}}{c}_{i}{t}^{i}$$ 

(6.1) 
where we assume that the reference epoch ${t}_{\mathrm{ref}}$, the middle of the time series, has already been subtracted from the time points.
${N}_{p}\ge 0$ is the degree of the polynomial, ${N}_{f}\ge 0$ is the number of detected frequencies, and ${N}_{h}(n)\ge 1$
is the number of significant harmonics of frequency ${f}_{n}$. This multifrequency harmonic model includes a loworder polynomial trend and n frequencies, each with k associated harmonics.