## Friday, May 24, 2019

### Logic Then and Now

This week I attended the Association of Symbolic Logic North American Annual Meeting in New York City, giving a talk on P v NP.

First I must share the announcement that ASL member Tuna Antinel of Lyon 1 University has been arrested in Turkey for his political beliefs. This website (English version) has details and how to show your support.

I last attended the ASL annual meeting at Notre Dame in 1993 as a young assistant professor. Back then I talked about then recent work using a special kind of generic oracle to make the Berman-Hartmanis isomorphism conjecture true. I remember someone coming up to me after the talk saying how excited they were to see such applications of logic. I'm not a theoretical computer scientist, I'm a applied logician.

I asked at my talk this year and maybe 2-3 people were at that 1993 meeting. The attendance seemed smaller and younger, though that could be my memory playing tricks. I heard that the 2018 meeting in Macomb, Illinois drew a larger crowd. New York is expensive and logicians don't get large travel budgets.

Logic like theoretical computer science has gotten more specialized so I was playing catch up trying to follow many of the talks. Mariya Soskova of Wisconsin talked about enumeration degrees that brought me back to the days I sat in logic classes and talks at the University of Chicago. A set A is enumeration reducible to B if from an enumeration of B you can compute an enumeration of A and Mariya gave a great overview of this area.

I learned about the status of an open problem for Turing reducibility: Is there a non-trivial automorphism of the Turing Degrees? A degree is the equivalence class where each class are the languages all computably Turing-reducible to each other. So the question asks if there is a bijection f mapping degrees to degrees, other than identity, that preserves reducibility or lack thereof.

Here's what's known: There are countably many such automorphisms. There is a definable degree C in the arithmetic hierarchy, such that if f(C) = C then f is the identity. Also if f is the identity on all the c.e.-degrees (those equivalence classes containing a computably enumerable set), then f is the identity on all the degrees. Still open if there is more than one automorphism.