1. Galton, F.: Problem 4001. Educ. Times 1, 17 (1873)
2. Watson, H.W.: Solution to problem 4001. Educ. Times 1, 115–116 (1873)
3. Galton, F., Watson, H.W.: On the probability of extinction of families. J. R. Anthropol. Inst. 4, 138–144 (1874)
4. Steffensen, J.F.: On Sandsynligheden for at Afkommet udder, Matem.Tiddskr.B, 19–23 (1930)
5. Steffensen, J.F.: Deux problèmes du calcul des probabilités. Ann. Inst. Henri Poincaré 3, 319–344 (1933)