Comparison of optical and X-ray PDS of MAXIJ1820+070. The top and bottom panels show the NICER PDS, while the middle panels show the Iqueye PDS. The left panels correspond to the data taken around 18 April 2018, and those on the right correspond to the data taken around 8 June 2018. As no overlapping observations are available, we used for comparison the X-ray observations taken before and after the optical observations. The vertical red lines correspond to the central frequency of the QPOs observed in the optical PDS. In both periods the frequency of the most prominent QPO in the X-rays is in good agreement with the frequency of the second QPO in the optical band.