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@article{CM_1968__20__29_0, author = {Feferman, Solomon}, title = {Persistent and invariant formulas for outer extensions}, journal = {Compositio Mathematica}, pages = {29--52}, publisher = {Wolters-Noordhoff Publishing}, volume = {20}, year = {1968}, mrnumber = {230610}, zbl = {0162.01601}, language = {en}, url = {http://geodesic.mathdoc.fr/item/CM_1968__20__29_0/} }
Feferman, Solomon. Persistent and invariant formulas for outer extensions. Compositio Mathematica, Tome 20 (1968), pp. 29-52. http://geodesic.mathdoc.fr/item/CM_1968__20__29_0/