Riedwyl, Hans (1990). "Rudolf Wolf's Contribution to the Buffon Needle Problem (an Early Monte Carlo Experiment) and Application of Least Squares". The American Statistician. 44 (2): 138–139. doi:10.2307/2684154. JSTOR2684154.
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Wolf, R. (1852). "Neue Untersuchungen über die Periode der Sonnenflecken und ihre Bedeutung" [New investigations regarding the period of sunspots and its significance]. Mittheilungen der Naturforschenden Gesellschaft in Bern (in German). 255: 249–270. Wolf's estimates of the solar cycle's period appear on p. 250 and p. 251.
Riedwyl, Hans (1990). "Rudolf Wolf's Contribution to the Buffon Needle Problem (an Early Monte Carlo Experiment) and Application of Least Squares". The American Statistician. 44 (2): 138–139. doi:10.2307/2684154. JSTOR2684154.