New reconstruction of the sunspot group numbers since 1739 using direct calibration and “backbone” methodsTheodosios Chatzistergos, Ilya G. Usoskin, Gennady A. Kovaltsov, Natalie A. Krivova and Sami K. SolankiA&A, 602 (2017) A69DOI: https://doi.org/10.1051/0004-6361/201630045
