S. Feferman and G. Kreisel. Persistent and invariant formulas relative to theories of higher order. Bulletin of the American Mathematical Society, vol. 72 (1966), pp. 480–485. - Solomon Feferman. Persistent and invariant formulas for outer extensions. Logic and foundations of mathematics, Dedicated to Prof. A. Heyting on his 70th birthday, Wolters-Noordhoff Publishing, Groningen1968, pp. 29–52; also Compositio mathematica, vol. 20 (1968), p. 29–52.
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