順序数定義可能集合
Gödel, Kurt (1965) [1946], “Remarks before the Princeton Bicentennial Conference on Problems in Mathematics”, in Davis, Martin, The undecidable. Basic papers on undecidable propositions, unsolvable problems and computable functions, Raven Press, Hewlett, N.Y., pp. 84–88, ISBN 978-0486432281, MR0189996
Kunen, Kenneth (1980), Set theory: An introduction to independence proofs, Elsevier, ISBN 978-0-444-86839-8