@article{Clette:1431,
      author        = "Clette, F. and Berghmans, D. and Vanlommel, P. and
                       Koeckelenbergh, A. and Van der Linden, R.A.M. and
                       Robbrecht, E.",
      title         = "{From the Wolf Number to the International Sunspot Index.
                       25 Years of SIDC}",
      month         = "Sep",
      year          = "2006",
}