Yu. Baryshnikov, Bell Labs Thursday, September 16 2:30pm Hard disks in a box: topology and complexity ABSTRACT Configuration spaces of hard disks (or balls) in a box support archetypal dynamical systems of statistical physics. Yet, we still know precious little about the structure of these space (say are they connected, for low densities?). This talk will be dealing with their topology. Joint work with M. Kahle, IAS.
 Adrian Ocneanu, Penn State Thursday, September 23 2:30pm Symmetry, regular solids and quantum field theory ABSTRACT We discuss the structures of 3-dimensional space motions and symmetry and their relation to 4-dimensional regular solids as illustrated by the Octacube sculpture. We present applications of these and of higher symmetry structures in models of quantum field theory.
 David Futer, Temple University Thursday, September 30 2:30pm The rental harmony theorem ABSTRACT The rental harmony theorem (proved by Francis Su a decade ago) is quite possibly the most practically useful theorem that I have come across. Suppose that n housemates are renting a house that has n unequal rooms. The theorem says that under mild hypotheses, there is a way to partition the total rent into rents for the individual rooms, such that every housemate will prefer a different room. Furthermore, it provides an algorithm to find this harmonious partition of the rent. I will explain the theorem and its proof.
 Richard Kenyon, Brown University Thursday, October 7 2:30pm Tilings with rational polygons ABSTRACT In 1903, Dehn showed that an aXb rectangle can be tiled by squares if and only if a/b is rational. We generalize this as follows. A convex polygon P can be tiled by rational polygons if and only if P is rational. Here by rational polygon we mean a polygon whose side length ratios are rational. The proof uses the notions of the signature of a quadratic form, and of rational linear maps from R to R, both of which we will introduce as necessary.
 Marl Sapir, Vanderbilt University Thursday, October 14 2:30pm Amenability of the Thompson group $F$ and some elementary properties of $Z^n$ ABSTRACT I will explain the notion of amenability of groups introduced by von Neumann in 1929., and define the R. Thompson group $F$ using diagram groups. I will formulate some elementary but still not proved properties of $Z^k$ (the standard cubical lattice in $R^k$) which, if true, would shed some light on the still open question about amenability of $F$. This is a joint work with A. Dranishnikov.
 Svetllana Jitomirskaya, University of California Irvine Thursday, October 21 11:15am Behind the Hofstadter's butterfly: the competition between order and chaos ABSTRACT Hofstadter's butterfly was maybe the first beautiful fractal produced in a numerical experiment (in 1976, before the word fractal came into existence). We will discuss some interesting features of the models behind it, called quasiperiodic operators, an area of strong interplay of ergodic theory, dynamical systems, probability, functional and harmonic analysis. Quasiperiodic operators feature a fascinating competition between randomness (ergodicity) and order (periodicity), that is often resolved on a deep arithmetic level. The interest in those models was enhanced by strong connections with some major discoveries in physics, such as integer quantum Hall effect, experimental quasicrystals, quantum chaos theory, as well as graphene. Quasiperiodic operators provide central or important models for all four. We will not talk much about physics but will touch upon a related question of how the difference between rationals and irrationals can play a role in nature.
 Kenneth Gross, University of Vermont Thursday, November 4 2:30pm Symmetry: From Triangles to Quantum Physics ABSTRACT Symmetry is a fundamental concept in nature and to the human mind. It occurs everywhere, from patterns of wallpaper, to the paintings of the great masters, to the structure of poetry and music, and throughout science. The purpose of this presentation is to explain the mathematical formulation of the concepts of symmetry and invariance, and to provide an indication of their breadth of applications. The presentation assumes familiarity with complex numbers, matrices, and calculus.
 Ron Perline, Drexel University Thursday, November 11 2:30pm Some reflections on mirrors ABSTRACT Some simple geometry can make surprisingly important contributions to aspects of mirror design. I will talk about three mirror designs and their simple associated mathematics: rectifying mirrors, equiareal mirrors, and sideways mirrors. Basic differential equations theory, vector calculus and some geometry are the only pre-requisites.
 Richard Schwartz, Brown University Thursday, November 18 2:30pm Outer Billiards, Polytope Exchange Maps, and Renormalization ABSTRACT Outer billiards is a simple dynamical system, defined relative to a convex shape in the plane. In my talk I will explain some connections between polygonal outer billiards, higher dimensional polytope exchange maps, and renormalization. I will exploit these connections to get a fairly complete understanding of outer billiards defined relative to the Penrose kite.
 Robert Finn, Stanford Univrsity Thursday, December 2 2:30pm Capillary surfaces ABSTRACT It would be hard to get through a day without encountering surfaces separating distinct adjacent fluids. In a grotesque accident of history, all such things have come to be labeled capillary surfaces (capillus = hair in Latin). You encounter them whenever you drink a glass of water or wash your face, or sing in the rain or enjoy a candlelit dinner or see a spiderweb. They appear in all sizes, from microscopic to planetary and beyond. Experience has let us adapt to everyday occurrences, but the needs of quantitative theory lead to challenges that defy traditional methods. The formal equations have beautiful geometric form, but almost no exact solutions are known, and surprising things happen that have yet to be clarified. One of them is a striking relation to billiards theory. Some of the simplest problems were solved in approximate form during the 19th century. These “solutions” are still routinely used by engineers as models for general behavior, however recent discoveries (in some of which undergraduate students participated) have shown that such procedures can lead to major errors. I will discuss the state of the theory, with emphasis on current unsolved problems.
 A. Petrunin, Penn State Thursday, October 28 2:30pm Alexandrov's theorem on development of polyhedron ABSTRACT