1. See K. Härtig, J. Surányi: Periodica Math. Hung.
6 (1975), pp. 235–240.
2. In addition to his articles, he wrote two books that thoroughly deal with this subject: Geometrie der Zahlen (Leipzig, 1897) and Diophantische Approximationen (Leipzig, 1907). A modern treatment is given in J.W. S. Cassels: An Introduction to the Geometry of Numbers (Springer, 1959), and further in the encyclopedic work P. Gruber, C.G. Lekkerkerker: Geometry of numbers (North Holland, 1987).
3. See Mat és Fiz. Lapok
50. (1943), pp. 182–183, problem 12 (in Hungarian).
4. G. Pick: Lotos Prag. (2) 19 (1900), pp. 311–319.
5. G. L. Alexanderson, J. Pedersen: The Oregon Math. Teacher, 1985. The presented format of the proof was given by Árpád Somogyi, who found it independently of Pólya.