@article{AMLG_1965__8_137783,
     author = {Boykan Pour-El and Marian and Hilary Putnam},
     title = {Recursively enumerable classes and their application to recursive sequences of formal theories.},
     journal = {Archiv f\"ur mathematische Logik und Grundlagenforschung},
     pages = {104--121},
     publisher = {mathdoc},
     volume = {8},
     year = {1965},
     zbl = {0242.02046},
     url = {http://geodesic.mathdoc.fr/item/AMLG_1965__8_137783/}
}