The equivalence of two definitions of elementary formal system
Compositio Mathematica, Tome 20 (1968), pp. 13-20 
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     author = {Curry, Haskell B.},
     title = {The equivalence of two definitions of elementary formal system},
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     year = {1968},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {20},
     mrnumber = {238671},
     zbl = {0167.01002},
     language = {en},
     url = {https://www.numdam.org/item/CM_1968__20__13_0/}
}
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Curry, Haskell B. The equivalence of two definitions of elementary formal system. Compositio Mathematica, Tome 20 (1968), pp. 13-20. https://www.numdam.org/item/CM_1968__20__13_0/

H.B. Curry [CFS]. Calculuses and formal systems. Dialectica 12: 249-273 (1958). [FML]. Foundations of Mathematical Logic. New York, McGraw-Hill Book Co., 1963. | Zbl | MR

H.B. Curry and Robert Feys [CLg.I] Combinatory Logic, vol. I. Amsterdam, North-Holland Publishing Co., 1958. | Zbl | MR

Emil L. Post [IGT]. Introduction to a general theory of elementary propositions. Amer. J. Math. 43: 163-185 (1921). | MR | JFM

Raymond M. Smullyan [TFS]. Theory of Formal Systems. Princeton, N.J., Princeton University Press, 1961. | Zbl | MR