# New directions in reverse mathematics

## Damir Dzhafarov

### University of Connecticut

Mathematics today benefits from having “firm foundations,” by which we usually mean a system of axioms sufficient to prove the theorems we care about. But given a particular theorem, can we specify precisely which axioms are needed to derive it? This is a natural question, and also an ancient one: over 2000 years ago, the Greek mathematicians were asking it about Euclid’s geometry. Reverse mathematics provides a modern approach to this kind of question. A striking fact repeatedly demonstrated in this area is that the vast majority of mathematical propositions can be classified into just five main types, according to which set-existence axioms are needed to carry out their proofs. But more recently, a growing number of principles falling outside this classification have emerged, whose logical strength is more difficult to understand. These turn out to include many important mathematical results, such as various combinatorial problems related to Ramsey’s theorem, and several set-theoretic equivalents of the axiom of choice. I will discuss some of these “irregular” principles, and some new approaches that have arisen from trying to understand why their strength is so different from that of most other theorems. In particular, this investigation reveals new connections between different mathematical areas, and exposes the rich and complex combinatorial and algorithmic structure underlying mathematics as a whole.

Damir Dzhafarov studies computability theory and reverse mathematics. He received his doctorate in 2011 from the University of Chicago, as a student of Profs. Robert Soare, Denis Hirschfeldt, and Antonio Montalban, and then held an NSF Postdoctoral Fellowship at Notre Dame University and at the University of California-Berkeley. In 2013 he joined the mathematics faculty of the University of Connecticut.