Discrete correlation function between the daily γ-ray flux and R-band flux density light curves shown in Fig. 16 before (left) and after (right) JD = 2   455   450 for a comparison with the Jorstad et al. (2011) results (see the text). The blue squares represent the case where the light curves are stopped at JD = 2   455   763 to avoid the influence of the optical flare without any significant γ-ray counterpart at JD ~ 2   455   767.