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Foundational and mathematical uses of higher types
 REFLECTIONS ON THE FOUNDATIONS OF MATHEMATICS: ESSAY IN HONOR OF SOLOMON FEFERMAN
, 1999
"... In this paper we develop mathematically strong systems of analysis in higher types which, nevertheless, are prooftheoretically weak, i.e. conservative over elementary resp. primitive recursive arithmetic. These systems are based on noncollapsing hierarchies ( n WKL+ ; n WKL+ ) of principles ..."
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Cited by 11 (4 self)
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In this paper we develop mathematically strong systems of analysis in higher types which, nevertheless, are prooftheoretically weak, i.e. conservative over elementary resp. primitive recursive arithmetic. These systems are based on noncollapsing hierarchies ( n WKL+ ; n WKL+ ) of principles which generalize (and for n = 0 coincide with) the socalled `weak' König's lemma WKL (which has been studied extensively in the context of second order arithmetic) to logically more complex tree predicates. Whereas the second order context used in the program of reverse mathematics requires an encoding of higher analytical concepts like continuous functions F : X ! Y between Polish spaces X;Y , the more exible language of our systems allows to treat such objects directly. This is of relevance as the encoding of F used in reverse mathematics tacitly yields a constructively enriched notion of continuous functions which e.g. for F : IN ! IN can be seen (in our higher order context)
A note on Goodman's theorem
, 1997
"... Goodman's theorem states that intuitionistic arithmetic in all finite types plus full choice, HA ! + AC, is conservative over firstorder intuitionistic arithmetic HA. We show that this result does not extend to various subsystems of HA ! , HA with restricted induction. 1 Introduction Let EHA ..."
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Goodman's theorem states that intuitionistic arithmetic in all finite types plus full choice, HA ! + AC, is conservative over firstorder intuitionistic arithmetic HA. We show that this result does not extend to various subsystems of HA ! , HA with restricted induction. 1 Introduction Let EHA ! denote the system of extensional intuitionistic arithmetic and HA ! its `neutral' variant as defined in [10]). 1 EPA ! and PA ! are the corresponding theories with classical logic. PA (resp. HA) is firstorder Peano arithmetic (resp. its intuitionistic version) with all primitive recursive functions. T denotes the set of finite types. The schema AC of full choice is given by S ae;ø 2T AC ae;ø , where AC ae;ø : 8x ae 9y ø A(x; y) ! 9Y ø(ae) 8x ae A(x; Y x) and A is an arbitrary formula of L(HA ! ). We also consider a restricted (arithmetical) form of AC: AC 0;0 ar : 8x 0 9y 0 A(x; y) ! 9f 0(0) 8x 0 A(x; fx); where A contains only quantifiers of type...