Productive sets. Dekker, J. C. E. Transactions of the American Mathematical Society, 78:129–149, 1955.
Productive sets [link]Paper  doi  bibtex   
@ARTICLE{Dek55,
  author = {Dekker, J. C. E.},
  title = {Productive sets},
  journal = {Transactions of the American Mathematical Society},
  year = {1955},
  volume = {78},
  pages = {129--149},
  doi = {10.2307/1992952},
  file = {Dek55.pdf:Dek55.pdf:PDF},
  issn = {0002-9947},
  mrclass = {02.0X},
  mrnumber = {67049},
  mrreviewer = {G. Kreisel},
  url = {https://doi.org/10.2307/1992952}
}

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