@misc{indiciaed35e15849a1e,
  title = {On equations with sets as unknowns.},
  author = {P Erdös and S Ulam},
  year = {1968},
  doi = {10.1073/pnas.60.4.1189},
  url = {https://pubmed.ncbi.nlm.nih.gov/16591682/},
  note = {Source identifier: 16591682}
}