Undergraduate Research Projects

Northwestern undergraduates have opportunities to explore mathematics beyond our undergraduate curriculum by enrolling in MATH 399-0 Independent Study, working on a summer project, or writing a senior thesis under the supervision of a faculty member. Below are descriptions of projects that our faculty have proposed.  Students interested in one of these projects should contact the project adviser. This should not be taken to be an exhaustive list of all projects that are availalbe, nor as a list of the only faculty open to supervising such projects. Contact the Director of Undergraduate Studies for additional guidance. These projects are only available to Northwestern undergraduates.

Combinatorial Structures in Symplectic Topology

Eric Zaslow

Symplectic and contact geometry describe the mathematics of phase space for particles and light, respectively.  They therefore are the mathematical home for dynamical systems arising from physics.  A noteworthy structure within contact geometry is that of a Legendrian surface, closely related to the wavefront of propagating light.  These subspaces sometimes have combinatorial descriptions via graphs.  The project explores how well the combinatorial descriptions can distinguish Legendrian surfaces, just as in knot theory one might explore whether the Jones polynomial can distinguish different knots. 

Prerequisites:  MATH 330-1 or MATH 331-1, MATH 342-0.
Recommended: MATH 308-0.

Complexity and Periodicity

Bryna Kra

The simplest bi-infinite sequences in {0,1}ℤ$\{0,1{\}}^{\mathbb{Z}}$ are the periodic sequences, where a single pattern is concatenated with itself infinitely often. At the opposite extreme are bi-infinite sequences containing every possible configuration of 0$0$'s and 1$1$'s. For periodic sequences, the number of substrings of length n$n$ is bounded, while in the second case, all substrings appear and so there are 2n$2^n$ substrings of length n$n$. The growth rate of the possible patterns is a measurement of the complexity of the sequence, giving information about the sequence itself and describing objects encoded by the sequence. Symbolic dynamics is the study of such sequences, associated dynamical systems, and their properties.

An old theorem of Morse and Hedlund gives a simple relation between this measurement of complexity and periodicity: a bi-infinite sequence with entries in a finite alphabet $\mathcal{A}$ is periodic if and only if there exists some n∈ℕ$n\in \mathbb{N}$ such that the sequence contains at most n$n$ words of length n$n$. However, as soon as we turn to higher dimensions, meaning a sequence in ℤd${\mathcal{A}}^{{\mathbb{Z}}^d}$ for some d≥2$d\ge 2$ rather than d=1$d=1$, the relation between complexity and periodicity is no longer clear.  Even defining what is meant by low complexity or periodicity is not clear. 

This project will cover what is known in one dimension and then turn to understanding how to generalize these phenomena to higher dimensions.  

Prerequisite: MATH 320-3 or MATH 321-3.

Finite Simple Groups

Ezra Getzler

Finite simple groups are the building blocks of finite groups. For any finite group G$G$, there is a normal subgroup H$H$ such that G/H$G/H$ is a simple group: the simple groups are those groups with no nontrivial normal subgroups.  The abelian finite simple groups are the cyclic groups of prime order; in this sense, finite simple groups generalize the prime numbers.

 One of the beautiful theorems of algebra is that the alternating groups An$A_n$ (subgroups of the symmetric groups Sn$S_n$) are simple for n≥5$n\ge 5$. In fact, A5$A_5$ is the smallest non-abelian finite simple group (its order is 60$60$).

Another series of finite simple groups was discovered by Galois. Let 𝔽$\mathbb{F}$ be a field.  The group SL2(𝔽)$S{L}_2(\mathbb{F})$ is the group of all 2×2$2\times 2$ matrices of determinant 1$1$. If we take 𝔽$\mathbb{F}$ to be a finite field, we get a finite group; for example, we can take 𝔽=𝔽p$\mathbb{F}={\mathbb{F}}_p$, the field with p$p$ elements. It is a nice exercise to check that SL2(𝔽p)$S{L}_2({\mathbb{F}}_p)$ has p3−p$p^3-p$ elements.

The center Z(SL2(𝔽p))$Z(S{L}_2({\mathbb{F}}_p))$ of SL2(𝔽p)$S{L}_2({\mathbb{F}}_p)$ is the set of matrices ±I$\pm I$; this has two elements unless p=2$p=2$. The group PSL2(𝔽)$PS{L}_2(\mathbb{F})$ is the quotient of SL2(𝔽)$S{L}_2(\mathbb{F})$ by its center Z(SL2(𝔽))$Z(S{L}_2(\mathbb{F}))$: we see that PSL2(𝔽p)$PS{L}_2({\mathbb{F}}_p)$ has order (p3−p)/2$(p^3-p)/2$ unless p=2$p=2$. It turns out that PSL2(𝔽2)$PS{L}_2({\mathbb{F}}_2)$ and PSL2(𝔽3)$PS{L}_2({\mathbb{F}}_3)$ are isomorphic to S3$S_3$ and A4$A_4$, which are not simple, but PSL2(𝔽5)$PS{L}_2({\mathbb{F}}_5)$ is isomorphic to A5$A_5$, the smallest nonabelian finite simple group, and PSL2(𝔽7)$PS{L}_2({\mathbb{F}}_7)$, of order 168$168$, is the second smallest nonabelian finite simple group. (When 𝔽$\mathbb{F}$ is the field of complex numbers, the group PSL2(ℂ)$PS{L}_2(\mathbb{C})$ is also very interesting, though of course it is not finite: it is isomorphic to the Lorentz group of special relativity.) 

The goal of this project is to learn about generalizations of this construction, which together with the alternating groups yield all but a finite number of the finite simple groups. (There are 26 missing ones called the sporadic simple groups that cannot be obtained in this way.) This mysterious link between geometry and algebra is hard to explain, but very important: much of what we know about the finite simple groups comes from the study of matrix groups over the complex numbers.

Prerequisite: MATH 330-3 or MATH 331-3.

Fourier Series and Representation Theory

Eric Zaslow

Fourier series allow you to write a periodic function in terms of a basis of sines and cosines.  One way to think of this is to understand sines and cosines as the eigenfunctions of the second derivative operator – so Fourier series generalize the spectral theorem of linear algebra in this sense.  There is another viewpoint that is useful:  periodic functions can be thought of as functions defined on a circle, which is itself a group.  The connection between group theory and Fourier series runs deeper, and this is the subject of this project.

Moving up a dimension, functions on a sphere can be described in terms of spherical harmonics.  While the sphere is not a group, it is the orbit space of the unit vector in the vertical direction.  Thus it can be constructed as a homogeneous space:  it is the group of rotations modulo the group of rotations around the vertical axis.  We can therefore access functions on the sphere via functions on the group of rotations.  The Peter-Weyl theorem describes the vector space of functions on the group in terms of its representation theory.  (A representation of a group is a vector space on which group elements act as linear transformations [e.g., matrices], consistent with their relations.)  The entries of matrix elements of the irreducible representations of the group play the role that sines and cosines did above.  Indeed, we can combine sines and cosines into complex exponentials and these are the sole entries of the one-by-one matrices (characters) representing the abelian circle group. 

Finally, we will connect spherical harmonics to polynomial functions relevant to geometric structures described in the Borel-Weyl-Bott theorem.  Students will explore many examples along with learning the foundations of the theory.

Prerequisites:  MATH 351-0 or MATH 381-0.