Fri Oct 9, 2015
Takayuki Kihara (JSPS Postdoctoral Fellow, Department of Mathematics, UC Berkeley)
Recursion Theoretic Methods in Descriptive Set Theory and Infinite Dimensional Topology
The notion of degree spectrum of a structure in computable model theory is defined as the collection of all Turing degrees of presentations of the structure. We introduce the degree spectrum of a represented space as the class of collections of all Turing degrees of presentations of points in the space. The notion of point degree spectrum creates a connection among various areas of mathematics including computability theory, descriptive set theory, infinite dimensional topology and Banach space theory.
Through this new connection, for instance, we construct a family of continuum many infinite dimensional Cantor manifolds with property C in the sense of Haver/Addis-Gresham whose Borel structures at an arbitrary finite rank are mutually non-isomorphic. This provides new examples of Banach algebras of real valued Baire class two functions on metrizable compacta, and strengthen various theorems in infinite dimensional topology such as Pol’s solution to Alexandrov’s old problem.
To prove our main theorem, an invariant which we call degree co-spectrum, a collection of Turing ideals realized as lower Turing cones of points of a Polish space, plays a key role. The key idea is measuring the quantity of all possible Scott ideals (omega-models of WKL) realized within the degree co-spectrum (on a cone) of a given space. This is joint work with Arno Pauly (University of Cambridge, UK).