# Calendar

Aug
18
Tue
Aug 18 @ 6:00 am – 8:00 am

Title: Group dualities: G-barrelled groups

Lecture held in Elysium.

Abstract
A natural notion in the framework of abelian groups are the group dualities. The most efficient definition of a group duality is simply a pair $(G, H)$, where $G$ denotes an abstract abelian group and $H$ a subgroup of characters of $G$, that is $H leq {rm Hom}(G, mathbb T)$. Two group topologies for $G$ and $H$ appear from scratch in a group duality $(G, H)$: the weak topologies $sigma(G, H)$ and $sigma (H, G)$ respectively. Are there more group topologies either in $G$ or $H$ that can be strictly related with the duality $(G, H)$? In this sense we shall define the term “compatible topology” and loosely speaking we consider the compatible topologies as members of the duality.

The locally quasi-convex topologies defined by Vilenkin in the 50′s form a significant class for the construction of a duality theory for groups. The fact that a locally convex topological vector space is in particular a locally quasi-convex group serves as a nexus to emulate well-known results of Functional Analysis for the class of topological groups.

In this talk we shall
deal with questions of the sort:
Under which conditions is there a locally compact topology in a fixed duality?
The same question for a metrizable, or a $k$-group topology.
We shall also introduce the $g$-barrelled groups, a class for which the Mackey-Arens Theorem admits an optimal counterpart. We study also the existence of $g$-barrelled topologies in a group duality $(G, H)$.

Joe Miller (University of Wisconsin)
Aug 18 @ 10:00 am – 11:00 am

Title: Redundancy of information: lowering effective dimension
by Joe Miller (University of Wisconsin) as part of Computability theory and applications

Abstract
A natural way to measure the similarity between two infinite
binary sequences X and Y is to take the (upper) density of their
symmetric difference. This is the Besicovitch distance on Cantor
space. If d(X,Y) = 0, then we say that X and Y are coarsely
equivalent. Greenberg, M., Shen, and Westrick (2018) proved that a
binary sequence has effective (Hausdorff) dimension 1 if and only if
it is coarsely equivalent to a Martin-Löf random sequence. They went
on to determine the best and worst cases for the distance from a
dimension t sequence to the nearest dimension s>t sequence. Thus, the
difficulty of increasing dimension is understood.

Greenberg, et al. also determined the distance from a dimension 1
sequence to the nearest dimension t sequence. But they left open the
general problem of reducing dimension, which is made difficult by the
fact that the information in a dimension s sequence can be coded (at
least somewhat) redundantly. Goh, M., Soskova, and Westrick recently
gave a complete solution.

I will talk about both the results in the 2018 paper and the more
recent work. In particular, I will discuss some of the coding theory
behind these results. No previous knowledge of coding theory is
assumed.

Aug
24
Mon
Fall semester instruction begins
Aug 24 all-day
Aug
25
Tue
Wayne Lewis (University of Hawaiʻi) @ Lecture held in Elysium
Aug 25 @ 6:00 am – 8:00 am

Title: Classification of Periodic LCA Groups of Finite Non-Archimedean Dimension
by Wayne Lewis (University of Hawaiʻi) as part of Topological Groups

Lecture held in Elysium.

Abstract
A periodic LCA group such that the $p$-components all have $p$-rank bounded above by a common positive integer are classified via a complete set of topological isomorphism invariants realized by an equivalence relation on pairs of extended supernatural vectors.

Remaining time will be devoted to a facilitated discussion on how things are going this fall/winter academic semester in your part of the world as you see it.

Andre Nies (University of Auckland)
Aug 25 @ 3:00 pm – 4:00 pm

Title: Discovering structure within the class of K-trivial sets
by Andre Nies (University of Auckland) as part of Computability theory and applications

Abstract
Joint work with Noam Greenberg, Joseph Miller, and Dan Turetsky

The K-trivial sets are antirandom in the sense that the initial segment complexity in terms of prefix-free Kolmogorov complexity K grows as slowly as possible. Chaitin introduced this notion in about 1975, and showed that each K-trivial is Turing below the halting set. Shortly after, Solovay proved that a K-trivial set can be noncomputable.

In the past two decades, many alternative characterisations of this class have been found: properties such as  being low for K, low for Martin-Löf (ML) randomness, and a basis for  ML randomness, which state in one way or the other that the set is close to computable.

Initially, the class of noncomputable K-trivials appeared to be amorphous. More recently, evidence of an internal structure has been found. Most of these results can be phrased in the language of a mysterious reducibility on the K-trivials which is weaker than Turing’s: A is ML-below B if each ML-random computing B also computes A.

Bienvenu, Greenberg, Kucera, Nies and Turetsky (JEMS 2016) showed that there an ML complete K-trivial set. Greenberg, Miller and Nies  (JML, 2019) established a dense hierarchy of subclasses of the K-trivials based on fragments of Omega computing the set, and each such subclass is an initial segment for ML. More recent results generalise these approaches using cost functions. They also show that each K-trivial set is ML-equivalent to a c.e. K-trivial.

The extreme lowness of K-trivials, far from being an obstacle, allows for methods which don’t work in a wider setting. The talk provides an overview and discusses open questions. For instance, is ML-completeness an arithmetical property of K-trivials?

Sep
1
Tue
Dona Strauss (University of Leeds) @ Lecture held in Elysium
Sep 1 @ 6:00 am – 8:00 am

Title: The Semigroup $beta S$
by Dona Strauss (University of Leeds) as part of Topological Groups

Lecture held in Elysium.

Abstract
If $S$ is a discrete semigroup, the semigroup operation on $S$ can be extended to a semigroup operation on its Stone–Čech compactification $beta S$. The properties of the semigroup $beta S$ have been a powerful tool in topological dynamics and combinatorics.

I shall give an introductory description of the semigroup $beta S$, and show how its properties can be used to prove some of the classical theorems of Ramsey Theory.

Patrick Lutz (UC Berkeley)
Sep 1 @ 10:00 am – 11:00 am

Title: Part 1 of Martin’s Conjecture for Order Preserving Functions
by Patrick Lutz (UC Berkeley) as part of Computability theory and applications

Abstract
Martin’s conjecture is an attempt to make precise the idea that the only natural functions on the Turing degrees are the constant functions, the identity, and transfinite iterates of the Turing jump. The conjecture is typically divided into two parts. Very roughly, the first part states that every natural function on the Turing degrees is either eventually constant or eventually increasing and the second part states that the natural functions which are increasing form a well-order under eventual domination, where the successor operation in this well-order is the Turing jump.

In the 1980′s, Slaman and Steel proved that the second part of Martin’s conjecture holds for order-preserving Borel functions. In joint work with Benny Siskind, we prove the complementary result that (assuming analytic determinacy) the first part of the conjecture also holds for order-preserving Borel functions (and under AD, for all order-preserving functions). Our methods also yield several other new results, including an equivalence between the first part of Martin’s conjecture and a statement about the Rudin-Keisler order on ultrafilters on the Turing degrees.

In my talk, I will give an overview of Martin’s conjecture and then describe our new results.

Sep
7
Mon
Labor day
Sep 7 all-day