Summary of the zDCF analyses on the long-term B2 1811+31 lightcurves: (a) zDCF of the long-term HE γ-ray and R-band lightcurves, (c) zDCF of the long-term HE γ-ray and 15 GHz lightcurves, (d) zDCF of the long-term R-band and 15 GHz lightcurves. The 1σ and 3σ bands are indicated in dark and light gray, respectively. Panel (b) shows the distribution of the reconstructed zDCF at zero time-lag for 10³ simulated uncorrelated HE γ-ray and optical lightcurves, whereas the red line marks the value reconstructed with the corresponding observed B2 1811+31 lightcurves in Figure 1.