@article{LeussuUsoskinValliappanetal.2017,
  author    = {R. Leussu and IIlya G. Usoskin and Senthamizh Pavai Valliappan and Andrea Diercke and Rainer Arlt and Carsten Denker and K. Mursula},
  title     = {Wings of the butterfly},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  series    = {sunspot groups for 1826-2015},
  volume    = {599},
  doi       = {10.1051/0004-6361/201629533},
  year      = {2017},
}