Chatzistergos, T., Usoskin, I. G., Kovaltsov, G. A., Krivova, N. A., & Solanki, S. K. (2017). New calibration of the group sunspot number series using a non-linear non-parametric method. Talk presented at DPG-Frühjahrstagung. Bremen. 2017-03-13 - 2017-03-17.