This semester the Logic Seminar will meet on Thursdays, 2:50 – 3:40 pm in Keller 402.

This Thursday we will have a (probably brief) organizational meeting.

Title: Some nonstandard remarks about Egyptian fractions

Abstract: An Egyptian fraction is a finite sum of fractions of the form $1/n$, where $n$ is a natural number. I’ll give simple proofs of some results about such fractions (also about Znám fractions). The proofs only require the compactness theorem from first order logic, though I’ll use the language of nonstandard analysis.

Speaker: Nayantara Bhatnagar (U. Delaware)

Title: Subsequence Statistics in Random Mallows Permutations

Abstract: The longest increasing subsequence (LIS) of a uniformly random permutation is a well studied problem. Vershik-Kerov and Logan-Shepp first showed that asymptotically the typical length of the LIS is 2sqrt(n). This line of research culminated in the work of Baik-Deift-Johansson who related this length to the GUE Tracy-Widom distribution.

We study the length of the LIS of random permutations drawn from the Mallows measure, introduced by Mallows in connection with ranking problems in statistics. We prove limit theorems for the LIS for different regimes of the parameter of the distribution. I will also describe some recent results on the longest common subsequence of independent Mallows permutations.

Relevant background for the talk will be introduced as needed.

This semester, the Analysis Seminar will meet on Tuesdays 3:30 – 4:20 pm in Keller 402.

Title :

What are the possible shapes of polynomial Julia sets?

Abstract :

Ever since the digital revolution and the emergence of computers, mathematicians have been fascinated by fractals, those geometric figures showing self-similar patterns and irregular structures. A well-known family of fractals introduced by Gaston Julia and Pierre Fatou in the early 20th century are the so-called Julia sets, obtained from the iteration of a polynomial of one complex variable. It has been known for a long time how rich and diverse the geometry of these Julia sets are, from Cantor sets to smooth curves as well as highly irregular figures.

But what exactly are the possible shapes of polynomial Julia sets? This question, raised by Bill Thurston shortly before he passed away, has a rather surprising answer : they can have any shape, except some trivial topological obstructions. In this talk, I will present the ideas underlying the proof of this result, which gives an explicit construction. In particular, we will see how potential theory comes into play. I will also discuss some related computational aspects.

Title: A Simple Proof of a Theorem of Woodin

Abstract: In a similar spirit as my talk last semester about computing
and non-standard models, I will relay Joel David Hamkins’ new proof of a
theorem of Woodin: that there is a function that enumerates any finite
set (if computed in the correct model M of arithmetic), and which can
enumerate any extension of that set (if run in the correct end-extension
of M).

Speaker: Monique Chyba

Title: Is control theory loosing control?

Abstract: We live in an era of exciting scientific advances such as discovering new planets and black holes far away in the universe or gaining a better understanding of our own biological system. Unsurprisingly, mathematics plays a dominant role in almost all of them. Control theory models, analyzes and synthesizes the behavior of dynamical systems. Those systems are described by sets of ordinary differential equations that include an additional parameter referred to as the ‘control’. It can be viewed as the ship’s wheel of the system in analogy to the navigation of a boat. A vast area of work takes place in optimal control theory. Indeed, since by using different controls we can achieve the same goal, optimization with respect to a given cost such as energy or time becomes a primary interest. I will present three specific examples to illustrate the field of control theory and its current limitations that call for an innovative way of thinking.