Cuts in models of PA and independence results
Diversity in elementary cuts
Let M⊨PA be countable and recursively saturated.
Smorybski proved that here are 2ℵ0 theories of pairs (M,K), such that K is not semiregular in M.
Kossak proved that there are 2ℵ0 theories of pairs (M,K), such that K is strong in M.
Problem: How many theories of pairs (M,K) are there, such that K is semiregular, but not regular in M.
By recent results of Kaye and Tin Lok Wong, every countable recursively saturated M has K and L as in the question, such that (M,K)≇(M,L).
References:
Smoryński, C. Elementary extensions of recursively saturated models of arithmetic. Notre Dame J. Formal Logic 22 (1981), no. 3, 193–203.
Smoryński, C. A note on initial segment constructions in recursively saturated models of arithmetic. Notre Dame J. Formal Logic 23 (1982), no. 4, 393–408
Kossak, Roman A note on satisfaction classes. Notre Dame J. Formal Logic 26 (1985), no. 1, 1–8.
Kaye, Richard; Wong, Tin Lok Truth in generic cuts. Ann. Pure Appl. Logic 161 (2010), no. 8, 987–1005.