Duffin, R. J.; Schaeffer, A. C. (1941). «Khintchine's problem in metric diophantine approximation». Duke Math. J.8 (2): 243–255. doi:10.1215/S0012-7094-41-00818-9.
Duffin, R. J.; Schaeffer, A. C. (1941). «Khintchine's problem in metric diophantine approximation». Duke Math. J.8 (2): 243–255. doi:10.1215/S0012-7094-41-00818-9.