Persistent and invariant formulas for outer extensions Solomon Feferman — 1968 Compositio Mathematica
Two notes on abstract model theory. I. Properties invariant on the range of definable relations between structures Solomon Feferman — 1974 Fundamenta Mathematicae
Two notes on abstract model theory. II. Languages for which, the set of valid sentences is semi-invariantly implicitly definable Solomon Feferman — 1975 Fundamenta Mathematicae