Close Computer $K$-index estimation by a new linear-phase, robust, non-linear smoothing method,
Hattingh, M., L. Loubser, D. Nagtegaal (1989) (1989),
Geophysical Journal International, 99, 3 p. 533–547,
10.1111/j.1365-246x.1989.tb02038.x.