1431 20160610110241.0 SIDCimport-1719 Clette, F. 2006 2006-09-xx From the Wolf Number to the International Sunspot Index. 25 Years of SIDC Berghmans, D. Vanlommel, P. Koeckelenbergh, A. Van der Linden, R.A.M. Robbrecht, E. 2nd International Symposium on Space Climate, Sinaia, Romania, 13/6/2006 Invited CTALKINVI