Research
[click to expand/collapse]
Topics: My primary interests lie at the intersection of
number theory and algebra, particularly understanding and discovering
algebraic structures in arithmetic, and connecting different types
of structures. Much of this is done with group theory,
representation theory, and/or harmonic analysis.
I've also done some things with graph theory, combinatorial
optimization and spectral geometry.
Links in green are from conference proceedings.
Please contact me for a copy of any paper you cannot
download.

Congruences for modular forms mod 2 and quaternionic Sideal classes
Canadian Journal of Mathematics, to appear
We use quaternionic modular forms to prove various congruences mod 2
between modular forms with differing AtkinLehner eigenvalues. The proofs
are related to the distribution of AtkinLehner signs and
the notion of quaternionic Sideal classes.

Refined dimensions of cusp forms, and equidistribution and bias of signs
Submitted (revised Apr 19, 2017)
We give dimensions of new spaces of squarefree level with prescribed
AtkinLehner eigenvalues or global root numbers, and find these signs are
equidistributed with a strict bias in the weight but perfectly equidistributed
in the level.

Distinguishing finite group characters and refined localglobal phenomena,
with Nahid Walji
Acta Arithmetica, to appear
We study the question of how often two finite group characters can agree,
and use this to say how many Euler factors of distinct primitive Artin Lfunctions
can agree in degree 2 or 3.
 The JacquetLanglands
correspondence, Eisenstein congruences, and integral Lvalues in
weight 2
Mathematical Research Letters,
to appear
We use the JacquetLanglands correspondence to generalize
congruence results of Mazur to nonprime level and to
Hilbert modular forms.

Periods and nonvanishing of central Lvalues for GL(2n),
with Brooke Feigon and David Whitehouse
Israel Journal of Mathematics, to appear
Under some local hypotheses, we prove a relation between the nonvanishing
of twisted central Lvalues for GL(2n) and periods over
GL(n, E), where E is a quadratic extension.
We also deduce analogous local results for supercuspidal representations.

Test vectors and central Lvalues for GL(2),
with Daniel File and Ameya Pitale
Algebra and Number Theory, Vol. 11, No. 2 (2017), 253318.
We extend work of Gross and Prasad on test vectors for GL(2) to cases of joint
ramification, and use this to generalize the Lvalue formula of my
IMRN paper with Whitehouse, an averagevalue formula of
FeigonWhitehouse, and a nonvanishing mod p result of
MichelRamakrishnan.

A comparison of automorphic and
Artin Lseries of GL(2)type agreeing at degree one primes,
with Dinakar Ramakrishnan
Contemporary Mathematics 664, Advances
in the Theory of Automorphic Forms and their Lfunctions
(Cogdell volume) (2016), 339350.
We show that if a 2dimensional Artin representation corresponds to an
automorphic
representation outside of a density 0 infinite set of places of a certain
form, then they correspond everywhere.
 Strong localglobal
phenomena for Galois and automorphic representations
RIMS Kôkyûroku 1973, Modular forms and automorphic representations
(2015), 120130.
An exposition of strong multiplicity one type results and refinements,
with an aim to explain the results of my Contemp. Math.
paper with Ramakrishnan.

Distinguishing graphs with zeta functions
and generalized spectra,
with Christina Durfee
[arXiv version]
Linear Algebra and its Applications 481 (2015), 5482.
A fundamental problem in graph theory is: when is a graph determined by its
spectrum? We investigate analogues of this question with zeta functions
in place of spectrum. Our work suggests that zeta functions are more
effective at
distinguishing graphs than the usual types of spectra studied.

How often should you clean your room?
with Krishnan (Ravi) Shankar
Discrete Mathematics & Theoretical Computer Science, Vol. 17, No. 1
(2015), 413442.
We introduce and study a combinatorial optimization problem motivated by
the question, "How often should you clean your room?"
See also popular writeups by Francis Woodhouse and
Jon Kujawa.

Local root numbers, Bessel models, and a conjecture of Guo and Jacquet,
with Masaaki Furusawa
Journal of Number Theory, Special Issue in Honor of
Steve Rallis,
Vol. 146
(2015), 150170.
We make a conjecture about the transfer of global SO(2)Bessel periods on
SO(2n+1) to GL(n, E) periods on GL(2n), where
E is the quadratic extension associated to the relevant form of SO(2),
and prove this when n = 2.

On central critical values of the degree four Lfunctions for GSp(4):
a simple trace formula,
with Masaaki Furusawa
Mathematische Zeitschrift, Vol. 277, No. 1 (2014), 149180.
As an application of our fundamental lemma I and III papers, we
prove a global Bessel identity for cuspidal automorphic representations of
GSp(4) which are supercuspidal at some component (plus some other local
hypotheses).
In particular, one obtains the global GrossPrasad Conjecture (a nonvanishing
theorem) for such representations.

On central critical values of the degree four Lfunctions for GSp(4): the fundamental lemma III,
with Masaaki Furusawa and Joseph Shalika
[preprint version]
Memoirs of the AMS, Vol. 225, No. 1057 (2013), x+134pp.
We extend the fundamental lemma from our
American Journal paper below, as well as one due to FurusawaShalika,
to the full Hecke algebra.
 Nonunique factorization and principalization
in number fields
Proceedings of the AMS, Vol. 139, No. 9 (2011), 30253038.
This describes the number and structure of irreducible factorizations of
an algebraic integer in the ring of integers of a number field, using
what were essentially Kummer's ideas.
 A relative trace formula for a compact Riemann surface,
with Mark McKee and Eric Wambach [errata, corrected version]
International Journal of Number Theory, Vol. 7, No. 2 (2011), 389429.
We interpret a relative trace formula on a hyperbolic compact Riemann
surface as a relation between the period spectrum and ortholength
spectrum of a given closed geodesic. This leads to various asymptotic
results on periods and ortholengths, as well as some simultaneous nonvanishing
results for two different periods.
 On central critical values of the degree four
Lfunctions for GSp(4): the fundamental lemma II, with Masaaki
Furusawa [preprint version]
American Journal of Mathematics, Vol. 133, No. 1 (2011), 197233.
We propose a different kind of relative trace formula than FurusawaShalika
to relate central spinor Lvalues to Bessel periods, and prove the
corresponding fundamental lemma. This relative trace formula has several
advantages over the previous ones.
 Central Lvalues and toric periods
for GL(2), with David Whitehouse
International Mathematics Research Notices (IMRN) 2009, No. 1 (2009), 141191.
Using Jacquet's relative trace formula, we get a formula for the central value
of a GL(2) Lfunction, refining results of Waldspurger.
[Old version (Nov. 13, 2006). This uses a simpler trace formula but is much less general.]
 Central
Lvalues and toric periods for GL(2)
RIMS Kôkyûroku 1617,
Automorphic Representations, Automorphic
Forms, Lfunctions and Related Topics (2008), 126137.
This is basically an extended introduction to the above paper, ending with
an outline of the relative trace formula approach to proving special value
formulas.
 Shalika periods on GL(2,D) and GL(4),
with Hervé Jacquet
[preprint version]
Pacific Journal of Mathematics, Vol. 233, No. 2 (2007), 341370.
Here we use a relative trace formula
to study period integrals, which yield results about exteriorsquare Lfunctions, and thus about transfer to GSp(4).

Transfer from GL(2,D) to GSp(4)
Proceedings of the 9th Autumn Workshop on Number Theory,
Hakuba, Japan (2006), 10pp.
These are notes from a talk explaining an application of my work with Jacquet
(above) to the question of transferring representations to GSp(4).
 Fourdimensional Galois representations of solvable type and automorphic forms
[abstract]
Ph.D. Thesis, Caltech, 2004, 81pp.
This contains the results in the two papers below, as well as a
classification of representations into GSp(4,C) of solvable type and
minor additional modularity results.
I wrote an informal note about
my thesis for
the layman
(by which I mean the mathematically or scientifically minded layman).
 Modularity
of hypertetrahedral
representations
[preprint version]
Comptes Rendus Mathematique, Vol. 339, No. 2 (2004), 99102.
This proves a new case of modularity
for fourdimensional Galois representations induced from a nonnormal
quartic extension. In particular, one obtains examples of modular
representations which are not essentially selfdual.

A symplectic case of Artin's conjecture
Mathematical Research Letters, Vol. 10, No. 4 (2003), 483492.
This gives a new case of Artin's conjecture in GSp(4,C) by establishing
the more general Langlands' reciprocity law in this case.
Undergraduate Research Supervised

Notes
[click to expand/collapse]
Notes on Number Theory and Representation Theory
 An introduction to
(principal) Lfunctions
and Lgroups, from a prep session for grad students
at TORA VIII
(19pp, Mar 2017).
 Sums of squares, sums of cubes, and
modern number theory, a sort of survey,
aimed at graduate students (27pp, 2015; revised Jun 2016).
 A brief overview of modular and automorphic
forms, aimed at graduate students (12pp, 2010; revised Jun 2016).
 My thesis for
the layman, an attempt to vaguely explain what I was working on to my
friends/undergrad students from Caltech (4pp, 2004).

Langlands, Tunnell, Wiles and Fermat. This is an attempt to very briefly
(and informally) explain how Lfunctions and automorphic forms/representations
are involved in the proof of Fermat's Last Theorem (4pp, 2004).
 Langlands' Conjecture for the Tetrahedral and
Octahedral Cases, a short introduction to Langlands' reciprocity
conjecture
with an exposition of the proof in the tetrahedral and octahedral cases, i.e.,
the LanglandsTunnell Theorem (7pp, 2002).
 Representations of S_3, A_4 and
S_4, a simple exercise to write the irreducible representations
as induced from onedimensionals (2pp).
Notes on Graph Theory and Algebraic Combinatorics
 Designs and Codes: Planes, Difference
Sets and Hadamard Matrices, from a talk to general math undergrads on
some research areas in algebraic combinatorics (5pp, revised 2009).
 A Brief Introduction to Coding Theory, aimed at introducing my Caltech Freshman Summer Institute
kids to a research project on Berlekamp's light bulb game (4pp, 2002). You
can see their work here.
 Difference
Sets with Group Characters, a quick proof, shown to me by John Dillon, of Maschietti's theorem, which gave us a new construction for an infinite class of difference sets (6pp, 1997).
