1. Bartels, J.: 1934, Twenty-seven day recurrences in terrestrial magnetic and solar activity, 1923-33. Terr. Magn. Atmos. Electr. 39, 201 – 202.
2. Bratsolis, E., Sigelle, M.: 1998, Solar image segmentation by use of mean field fast annealing. Astron. Astrophys. 131, 371 – 375.
3. Clette, F., Berghmans, D., Vanlommel, P., Van der Linden, R.A.M., Koeckelenbergh, A., Wauters, L.: 2007, From the Wolf number to the International Sunspot index: 25 years of SIDC. Adv. Space Res. 40, 919 – 928. doi: 10.1016/j.asr.2006.12.045 .
4. Cordeiro, M.: 2001, Algorithmic Patterns for Morphological Image Processing, Prentice-Hall, Englewood Cliffs.
5. Dougherty, E.R.: 1992, An Introduction to Morphological Image Processing, SPIE Optical Engineering Press.