Relativization with respect to formulas and its use in proofs of independence
Compositio Mathematica, Volume 20  (1968), p. 88-106
@article{CM_1968__20__88_0,
author = {Henkin, Leon},
title = {Relativization with respect to formulas and its use in proofs of independence},
journal = {Compositio Mathematica},
publisher = {Wolters-Noordhoff Publishing},
volume = {20},
year = {1968},
pages = {88-106},
zbl = {0155.02301},
mrnumber = {234812},
language = {en},
url = {http://www.numdam.org/item/CM_1968__20__88_0}
}

Henkin, Leon. Relativization with respect to formulas and its use in proofs of independence. Compositio Mathematica, Volume 20 (1968) , pp. 88-106. http://www.numdam.org/item/CM_1968__20__88_0/

A. Tabski, A. Mostowski and R. Robinson [A] Undecidable theories. North-Holland Publishing Company, 1953 (cf. especially pp. 24-25.) | MR 244048 | Zbl 0053.00401

K. Gödel [B] The consistency of the axiom of choice and of the generalized continuum-hypothesis with the axioms of set theory. Princeton University Press, 1940. | Zbl 0061.00902

G. Birkhoff [C] Lattice theory. American Mathematical Society, 1948. | MR 29876 | Zbl 0033.10103