@article{Callan07a,
  title = {Problem 11307},
  author = {David Callan},
  year = {2007},
  url = {http://www.jstor.org/stable/27642282},
  researchr = {https://researchr.org/publication/Callan07a},
  cites = {0},
  citedby = {0},
  journal = {The American Mathematical Monthly},
  volume = {114},
  number = {7},
  pages = {640},
}