@article{Clette:233,
      author        = "Clette, F. and Berghmans, D. and Vanlommel, P. and Van der
                       Linden, R.A.M. and Koeckelenbergh, A. and Wauters, L.",
      title         = "{From the Wolf number to the International Sunspot index:
                       25 years of SIDC}",
      year          = "2007",
      note          = "(Invited review)",
}