1. Zipf GK Human Behaviour and the Principle of Least Effort (Addison-Wesley, Reading, MA, 1949)
2. Frequency of earthquakes in California*
3. Neukum G Ivanov BA in Hazards Due to Comets and Asteroids edited by T. Gehrels (University of Arizona Press, Tucson, AZ, 1994) pp. 359 – 416
4. Avalanches and the distribution of solar flares