Cosmin Pohoata. Problem 11468. The American Mathematical Monthly, 116(10):940, 2009. [doi]
@article{Pohoata09b, title = {Problem 11468}, author = {Cosmin Pohoata}, year = {2009}, url = {http://www.jstor.org/stable/40391257}, researchr = {https://researchr.org/publication/Pohoata09b}, cites = {0}, citedby = {0}, journal = {The American Mathematical Monthly}, volume = {116}, number = {10}, pages = {940}, }