@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://www.numdam.org/item/CM_1968__20__29_0/}
}