Donald E. Knuth. Problem 10985. The American Mathematical Monthly, 110(1):58, 2003. [doi]
@article{Knuth03-1, title = {Problem 10985}, author = {Donald E. Knuth}, year = {2003}, url = {http://www.jstor.org/stable/3072348}, researchr = {https://researchr.org/publication/Knuth03-1}, cites = {0}, citedby = {0}, journal = {The American Mathematical Monthly}, volume = {110}, number = {1}, pages = {58}, }