The Dennes Room

Event Detail

Fri Jan 29, 2021
Zoom, 4:10–6 PM
Logic Colloquium
James Walsh (Cornell University)
Reducing omega-model reflection to iterated syntactic reflection

Two types of principles are commonly called “reflection principles” in reverse mathematics. According to syntactic reflection principles for T, every theorem of T (from some complexity class) is true. According to semantic reflection principles, every set belongs to some (sufficiently correct) model of T. We will present a connection between syntactic reflection and semantic reflection in second-order arithmetic: For any Pi^1_2 axiomatized theory T, every set is contained in an omega-model of T if and only if every iteration of Pi^1_1 reflection for T along a well-ordering is Pi^1_1 sound. There is a thorough proof-theoretic understanding of the latter in terms of ordinal analysis. Accordingly, this reduction yields proof-theoretic analyses of omega-model reflection principles. This is joint work with Fedor Pakhomov.

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