# Seminars and Colloquia

## Algebra Seminar

Apr 6

#### Algebra Seminar

01:20 pm

Exley Science Center Tower ESC 618

Cameron Hill, Wesleyan Some thoughts on probability measures as varieties. Abstract: There are special infinite structures (graphs and hyper-graphs) that reflect the asymptotic first-order properties of their finite induced substructures very, very accurately. Exactly what makes these infinite structures special can be described model-theoretically in terms super-simplicity or in terms of higher amalgamation properties of the finite substructures. Using machinery from functional analysis, I have proved that model-theoretic specialness yields certain nice probability measures with which you then perform your asymptotic analyses. I dont fully understand what these measures are really like, and I imagine that some algebraic geometry might provide a more concrete transformation from higher amalgamation properties to measures. I will describe a possible set up for this.

Feb 16

#### Algebra Seminar

01:20 pm

Exley Science Center Tower ESC 618

Wai Kiu Chan, Wesleyan Warings problem for integral quadratic forms Abstract : For every positive integer n , let g ( n ) be the smallest integer such that if an integral quadratic form in n variables can be written as a sum of squares of integral linear forms, then it can be written as a sum of g ( n ) squares of integral linear forms. As a generalization of Lagranges Four-Square Theorem, Mordell (1930) showed that g (2) = 5 and later that year Ko (1930) showed that g ( n ) = n + 3 when n 5. More than sixty years later, M.-H. Kim and B.-K. Oh (1996) showed that g (6) = 10, and later (2005) they showed that the growth of g ( n ) is at most an exponential of n . In this talk, I will discuss a recent improvement of Kim and Oh's result showing that the growth of g ( n ) is at most an exponential of $\sqrt{n}$. . This is a joint work with Constantin Beli, Maria Icaza, and Jingbo Liu.

Nov 3

#### Algebra Seminar

01:20 pm

Exley Science Center Tower ESC 618

Jonathan Huang, Wes A Macdonald formula for zeta functions of varieties over finite fields Abstract : We provide a formula for the generating series of the zeta function Z ( X , t ) of symmetric powers Sym n X of varieties over finite fields. This realizes Z ( X , t ) as an exponentiable motivic measure whose associated Kapranov motivic zeta function takes values in W ( R ) the big Witt ring of R = W ( ). We apply our formula to compute Z (Sym n X , t ) in a number of explicit cases. Moreover, we show that all -ring motivic measures have zeta functions which are exponentiable. In this setting, the formula for Z ( X , t ) takes the form of a MacDonald formula for the zeta function.

Sep 27

#### Algebra Seminar

04:15 pm

Exley Science Center Tower ESC 618

Bweong-Kweon Oh (Seoul National University): The number of representations of squares by integral quadratic forms Abstract: Let f be a positive definite integral ternary quadratic form and let r ( k , f ) be the number of representatives of an integer k by f . We say that the genus of f is indistinguishable by squares if for any integer n , r ( n 2 , f ) = r ( n , f ) for any quadratic form f in the genus of f . In this talk, we will give some examples of non trivial genera of ternary quadratic forms which are indistinguishable by squares. Also, we give some relations between indistinguishable genera by squares and a conjecture by Cooper and Lam, and we resolve their conjecture completely. This is a joint work with Kyoungmin Kim.

Sep 20

#### Algebra Seminar

04:15 pm

Exley Science Center Tower ESC 618

Christopher Rasmussen (Wes):A (Necessarily Incomplete) Introduction to Frobenioids Abstract: A man who knows a little is sometimes more dangerous than a man who knows nothing at all. In his approach to proving the ABC Conjecture, Mochizuki relies on the concept of a Frobenioid, which in his own words is ``a sort of a category-theoretic abstraction of the theory of divisors on [models of global fields].'' In the present talk, we will attempt to carefully introduce the notion of a Frobenioid and provide a small amount of context. Nothing will be assumed beyond a basic knowledge of category theory and some standard algebra.

Feb 5

#### Algebra Seminar

01:15 pm

Exley Science Center Tower ESC 618

Michael Wijaya, Dartmouth College: A function-field analogue of Conway's topograph Abstract: In "The Sensual (Quadratic) Form", Conway introduced a new visual method to display values of a binary quadratic form Q(x,y)=ax^2+bxy+cy^2 with integer coefficients. This topograph method, as he calls it, leads to a simple and elegant method of classifying integral binary quadratic forms and answering some basic questions about them. In this talk, I will present an analogue of Conway's topograph method for binary quadratic forms with coefficients in F_q[T], where q is an odd prime power. The constructions will take place on the Bruhat-Tits tree of SL(2), which is an analogue of the real hyperbolic plane.