## University Seminars: Logic Across Disciplines Archives

**Spring 2018**

**Title**:Working seminar about developing a formal system representing reasoning in chemistry**Speaker: ****Prof. Michele Friend**, Department of Philosophy.**Date and Time:** Thursday, April 27, 2018 11:00am–12:00noon**Place**: Phillips Hall (801 22nd Street), Room 736

**Title**:*Scott Ranks of Scattered Linear Orders***Speaker: **Rachael Alvir, University of Notre Dame**Date and Time:** Thursday, April 26, 2018 12:30PM-1::30 PM**Place**: Rome Hall(801 22nd Street), Room 351

**Abstract:** The logic *L*(omega1, omega) is obtained from regular finitary first-order logic by closing under countable conjunctions and disjunctions. There is a kind of normal form for such sentences. The Scott rank of a countable structure A is the least complexity of a sentence A of *L*(omega1, omega), which describes *A* up to isomorphism among countable structures. Every scattered linear order is associated with an ordinal known as its Hausdorff rank. We give sharp upper bounds on the Scott rank of a scattered linear order given its Hausdorff rank, along the way calculating some of the back-and-forth relations on this class. These results generalize previously obtained results on the Scott ranks of ordinals and Hausdorff rank 1 linear orders.

**Title**:*Detecting Nilpotence in Classes of Groups***Speaker: **Iva Bilanovic, GWU**Date and Time:** Friday, March 30, 2018 11:00AM-12::00 PM**Place**: Phillips Hall (801 22nd Street), Room 736

**Abstract:** Detecting an arbitrary Markov property is π2-hard in the class of recursively presented groups and is π1-hard in the class of computable groups. In other words, even when a computable description of a group is given we cannot algorithmically decide whether the group has some property. Certain properties attain even higher level of complexity. We will investigate nilpotence and precisely locate its undecidability in the arithmetical hierarchy.

**Title**: Structural Properties of Spectra and Omega-Spectra**Speaker:**Alexandra Soskova, Sofia University, Bulgaria

https://store.fmi.uni-sofia.bg/fmi/logic/asoskova/**Date and Time:** Thursday, March 22, 2018, 12:30-1:30 PM**Place**: Rome Hall (801 22nd Street), Room 531

**Abstract:** We consider the degree spectrum of a structure from the point of view of enumeration reducibility and omega-enumeration reducibility. We will give an overview of several structural properties of degree spectra and their co-spectra, such as a minimal pair theorem and the existence of quasi-minimal degrees for degree spectra and receive as a corollary some fundamental theorems in enumeration degrees. We will show that every countable ideal of enumeration degrees is a co-spectrum of a structure and if a degree spectrum has a countable base then it has a least enumeration degree. Next we investigate the omega-enumeration co-spectra and show that not every countable ideal of omega-enumeration degrees is an omega-co-spectrum of a structure.

**Title**: *Transforming Machine Learning Heuristics into Provable Algorithms: Classical, Stochastic, and Neural***Speaker:** Cheng Tang, GWU, Computer Science

https://sites.google.com/site/**Date and Time:** Friday February 23, 11:00-12:00 noon**Place**: Phillips Hall (801 22nd Street), Room 736

**Abstract:** A recurring pattern in many areas of machine learning is the empirical success of a handful of “heuristics”, i.e., any simple learning procedure favored by practitioners. Many of these heuristic techniques lack formal theoretical justification. For unsupervised learning, Lloyd's k-means algorithm, while provably exponentially slow in the worst-case, remains popular for clustering problems arising from different applications. For supervised learning, random forest is another example of a winning heuristic with many variants and applications. But the most prominent example is perhaps the blossoming field of deep learning, which is almost entirely composed of heuristics; the practical success of a deep learning algorithm usually relies on an experienced user skillfully and creatively combining heuristics. In this talk, I will discuss some of my thesis work in advancing the theoretical understanding of some of the most widely used machine learning heuristics.

**Title:** *Effective Ultraproducts and Their Applications***Speaker:** Rumen Dimitrov, Western Illinois University

http://www.wiu.edu/users/**Date and Time:** Thursday February 8, 12:30-1:30**Place:** Phillips Hall (801 22nd Street), Room 730

**Abstract: **We use cohesive (effectively indecomposable) sets to build nonstandard versions of the field of rational numbers. We study the isomorphism types of these models when the complements of the cohesive sets are computably enumerable. Using Koenigsmann’s work on Hilbert's Tenth Problem we give a new proof that these fields are rigid.

**Titl****e:** Topological Spaces of Orderings of Algebraic Structures**Speaker:** Jennifer Chubb, University of San Francisco and GWU

http://www.cs.usfca.edu/~jcchubb/**Date and Time:** Friday, February 2 2018 11:00 am-12 noon**Place:** Phillips Hall (801 22nd Street), Room 736

**Abstract: **A left- or bi- partial ordering of an algebraic structure is a partial ordering of the elements of the structure that is invariant under the structure acting on itself on the left or, respectively, both on the left and on the right. In this talk, we will consider the spaces of total left and bi-orderings of a group, how these spaces can be visualized as the paths of binary trees, and their computational and topological properties.

**Titl****e:** Encoding Noncomputable Sets into Orders on Computable Structures**Speaker:** Valentina Harizanov, GWU

http://home.gwu.edu/~harizanv/**Date and Time:** Friday, January 26, 2018 11:00 am-12 noon**Place:** Phillips 736

**Abstract: **We consider a structure with a binary operation, which admits orders that are invariant under the operation. The space of these orders is compact under a natural topology, and in many cases homeomorphic to the Cantor set. For such computable structures, including many groups, we show when it is possible to encode an arbitrary set into an order so that their Turing degrees are preserved.

**Fall 2017**

**Title:** An intuitive description of toposes, toposes as a description of intuitionism

**Speaker:** Tslil Clingman, Johns Hopkins University**Date and Time:** Thursday, November 30, 2017, 2:30pm-3:30pm

**Place:** Rome Hall (801 22nd Street), Room 771

**Abstract: ** ** **In this talk we will begin by examining, by way of example, the notion of a topos, a category exhibiting certain structural properties satisfied in particular by the familiar category of sets and functions but many strange and wonderful examples besides. We will then observe how the presence of certain structural properties naturally equips a topos with `internal' and `external' models intuitionistic propositional logic. From here we will explore the `Mitchell-Bènabou' language of a topos -- a framework which will allow us to redevelop much of the usual language of set theory internal to any topos. That is, we will see how we may make sense of forming objects of a topos via comprehensions, quantifiers and many other appropriate tools as in the case of "surjections(X,Y) = {f ∈ Y^X | ∀y∈Y ∃x∈X [f(x)=y]}" for the (object of) surjections from X to Y. That objects defined in this manner are in factthe ones we desired depends further on the semantics of internal language, the so termed Kripke-Joyal semantics of a topos. Time allowing we will develop this more fully and explore the relation this bears to the forcing arguments of Cohen.

**Title:** Gödel Index Sets of Computable Structures

**Speaker:** Valentina Harizanov, GWU**Date and Time:** Thursday, November 16 2017, 02:30pm-03:30pm

**Place:** Rome Hall 771

**Abstract: ** For a computable structure, we define its index set to be the set of all Gödel codes for computable isomorphic copies. We will show how to calculate precisely the complexity of the index sets for some familiar algebraic structures. We will further discuss the most recent results in this area.

**Title:** Classification and measure for algebraic fields

**Speaker:** Russell Miller, City University of New York__http://qcpages.qc.cuny.edu/~____rmiller/__**Date and Time:** Friday, November 10, 2017, 03:00pm-04:00pm

**Place:** Rome Hall 771

**Abstract: ** The algebraic fields of characteristic 0 are precisely the subfields of the algebraic closure of the rationals, up to isomorphism. We describe a way to classify them effectively, via a computable homeomorphism onto Cantor space. This homeomorphism makes it natural to transfer Lebesgue measure from Cantor space onto the class of these fields, although there is another probability measure on the same class, which seems in some ways more natural than Lebesgue measure. We will discuss how certain properties of these fields – notably, relative computable categoricity – interact with these measures: the basic result is that only measure-0-many of these fields fail to be relatively computably categorical. (The work on computable categoricity is joint with Johanna Franklin.)

**Title:** Computable Classification Problem

**Speaker:** Valentina Harizanov, GWU __http://home.gwu.edu/~harizanv/__**Date and Time:** Thursday, November 2, 2017, 02:30pm-03:30pm

**Place:** Rome Hall 771

**Abstract: ** The Scott Isomorphism Theorem says that for any countable structure *M* there is a sentence, in countable infinitary language, the countable models of which are exactly the isomorphic copies of *M*. Here, we consider a computable structure *A* and define its index set to be the set of all Gödel codes for computable isomorphic copies of *A*. We will present evidence for the following thesis. To calculate the precise complexity of the index set of A, we need a good description of A, using computable infinitary language, and once we have an optimal description, the exact complexity within a computability-theoretic hierarchy will match that of the description.

**Title:** Trees of orderings

**Speaker:** Jennifer Chubb, University of San Francisco __http://www.cs.usfca.edu/~jcchu bb/__

**Date and Time:**Friday, October 27, 2017, 04:15am-05:15pm

**Place:** Rome Hall 771

**Abstract: ** An ordering of an algebraic structure with identity can be often identified with the corresponding set of positive elements. For a given algebraic structure, we can organize the cones of all the admitted orderings on a tree. When the structure is computable, the tree can be constructed in an effective way. Topological properties of this space of orderings can provide insight into algorithmic properties of the orderings, and vice versa. In this talk, we will see how to construct these trees and what they can tell us.

**Title:** Orderings of algebraic structures

**Speaker:** Jennifer Chubb, University of San Francisco __http://www.cs.usfca .edu/~jcchubb/__

**Date and Time:** Thursday, October 19, 2017, 2:30pm-3:30pm

**Place:** Rome 771

**Abstract: ** A left- or bi- partial ordering of an algebraic structure is a partial ordering of the elements of the structure that is invariant under the structure acting on itself on the left or, respectively, both on the left and on the right. I will discuss algorithmic properties of the orderings admitted by a computable structures and their general properties, and describe some open problems.

**Logic-Topology Seminar**

**Title:** Mathathon II: Search for Interesting Torsion in Khovanov Homology

**Speaker:** Jozef Przytycki, GWU

http://home.gwu.edu/~przytyck/**Date and Time:** Thursday, October 5, 2017, 2:30-3:30pm

**Place:** Rome Hall (801 22nd Street), Room 771

**Abstract: ** We will describe the work of our Mathathon group (Sujoy Mukherjee, Marithania Silvero, Zhao Wang, Seung Yeop Yang), Dec. 2016–Jan. 2017, on torsion in Khovanov homology different from Z_2. Khovanov homology, one of the most important constructions at the end of XX century, has been computed for many links. However, computation is NP-hard and we are limited to generic knots of up to 35 crossings with only some families with larger number of crossings. The experimental data suggest that there is abundance of Z_2-torsion but other torsion seems to be rather rear phenomenon. The first Z_4 torsion appears in 15 crossing torus knot T(4,5), and the first Z_3 and Z_5 torsion in the torus knot T(5,6). Generally, calculations by Bar-Nathan, Shumakovitch, and Lewark suggest Z_p^k torsion in the torus knot T(p^k,p^k+1), p^k >3, but this has not yet been proven. We show, with Mathathoners, the existence of Z_n-torsion, n>3, for some infinite family of knots. The simplest of them is obtained by deforming the torus knot T(5,7) by a t_2k-moves. We also prove the existence of knots with other torsion, the largest being Z_2^23, so the cyclic group of over 8 millions elements. We combine computer calculations (and struggle with NP hardness) with homological algebra technique.

The talk will be elementary and all needed notions will be defined.