William J. Keith
Michigan Tech University Math Department
Tenure-track Assistant Professor



Teaching

My courses for the fall of 2016 are:

MA2330, Section 1    Linear Algebra   3:05pm - 3:55pm, MWF    Fisher 328    Canvas page
MA3210, Section 1    Intro to Combinatorics   8:05am - 9:20am, TR    Fisher 131    Canvas page

My office hours are MWF 12-1pm. My office is Fisher 316.
Outside of my office hours, students should get in touch with me using my MTU email, wjkeith [at] mtu.edu (no spaces). I am generally available at other times MWF if a student emails me well in advance. This and other class information is available on the syllabus for each class, which is also on the course webpages linked above. On Tuesdays and Thursdays after class I am typically involved in research activities and unavailable for student meetings.


Service

I currently serve on the Undergraduate Committee and help organize the Combinatorics Seminar.

Here is the Combinatorics Seminar Schedule for the current semester, with previous schedules back to the Spring of 2013. Seminars this semester run Thursdays 1-2pm in Fisher 325.


Research

My research is in combinatorics, specializing in partition theory and related q-series and identities.

For the standard outline of my research, please help yourself to a copy of my CV and publication list, both in pdf format. For more detail, I list below a few of my papers (and my thesis). Preprints of all my work are available on the arXiv.
Graduate students who find the topics of these papers interesting are warmly encouraged to discuss potential thesis work with me!

Selected Publications and Preprints

  • My thesis, generalizing two theorems in the literature on congruences for the full rank and on a theorem of Fine, both to more general modulus.

  • Proof of a conjectured q,t-Schröder identity. Discrete Mathematics, Volume 310, Issue 19, 6 October 2010, Pages 2489 - 2494. The k=2 case (one with interesting combinatorial interpretation) of a larger open conjecture by Chunwei Song related to the q,q limit of the q,t-Schröder theorem.

  • A Bijection for Partitions with initial repetitions. The Ramanujan Journal, February 2012, Volume 27, Issue 2, pp 163-167. A short paper with a bijective proof of another theorem of Andrews, proved with q-series techniques.

  • The 2-adic Valuation of Plane Partitions and Totally Symmetric Plane Partitions. Elec. Journ. of Comb., Volume 19 (2012), paper 48. Answering a question of Amdeberhan, Manna and Moll.

  • (Joint w/ Rishi Nath, CUNY-York) Partitions with prescribed hooksets. Journal of Combinatorics and Number theory, Volume 3 (1) 2011. The link goes to the preprint, since the journal is not as widely accessible.

  • A Ramanujan congruence analogue for Han's hook-length formula mod 5, and other symmetries. In submission; the link is the arXiv preprint. Raise the partition function to the power 1-b (or its reciprocal to b-1), and expand it as a power series in q with coefficients in the indeterminate b. The resulting polynomials have a large number of very pleasing symmetries.

  • Congruences for 9-regular partitions modulo 3. There seem to be a surprising number of congruences modulo 2 and 3 for partitions in which parts are not divisible by various values, and a topic of my current research is understanding why.

  • Restricted k-color partitions. Overpartitions are a hot current topic in partition research, and colored partitions are an old standby in the literature. Here I unify the two topics and ask about the combinatorial properties of the resulting objects.

  • (Joint with Fabrizio Zanello and his graduate student Samuel Judge.) On the density of the odd values of the partition function. One of the oldest and hardest questions in partition theory is whether it is true, as it seems to be, that half of all partition numbers are odd and half even. This paper conjectures and proves a family of formulas that relate the density of odd values of various powers of the partition function, and derives some conditional bounding results on those densities.

  • The part-frequency matrices of a partition. A generalization of Glaisher's map to all partitions suggests a new statistic for working with partition bijections and congruences.

  • (Joint with Donald Kreher and Dalibor Fronček.) A note on nearly platonic graphs. One of the fun things about teaching lower-division courses is that sometimes a challenging question can still pop up. When I was teaching an undergraduate combinatorics course, I found myself asking whether it was possible to draw a graph that was platonic except that one face was a different degree. It turns out this is impossible, and not trivial to show!

  • Proof of Xiong's conjectured refinement of Euler's partition theorem. One of the first theorems a student of partitions learns is that the number of partitions of n into odd parts equals the number of partitions of n into distinct parts. This theorem has been refined in many ways over the centuries; this paper proves a conjecture in that direction which someone else advanced recently.


    Graduate Students

    I am presently supervising J. T. Davies in research permutation statistics. Our motivating question: the major index is symmetric over some sets of pattern-avoiding permutations in Sn with fixed descent number, and (maj, des) form a Mahonian pair. Are there conditions analogous to pattern avoidance (and hopefully equally interesting) for other pairs such as (den, exc) which are known to be Mahonian but are not distributed symmetrically over pattern-avoidance classes?


    Ongoing Research

    These are a few of the ongoing research questions which interest me. I am always happy to receive comments from interested colleagues, and would be pleased to collaborate with someone who has useful ideas in these directions. Graduate students considering combinatorics who find some of these questions interesting are encouraged to contact me as well.

    1.) I continue with great interest to study more about the polynomials described in the "Ramanujan congruence analogue" paper, which I think are fascinating combinatorial objects with properties that deserve exploration.

    2.) I am interested in m-regular partitions, especially their low-modulus congruences. Related to this, I would like to show properties of singular overpartitions related to known theorems such as the Pak-Postnikov (m,c) theorem.

    3.) I have recently been studying Kanade and Russell's very curious conjectures on asymmetric versions of the G¨auto;llnitz-Gordon theorem.

    4.) Of the famous problems that interest me, I would name the Borwein Conjecture, on the positivity of coefficients of a series defining a certain weighted sum over specialized partitions, and Lehmer's Conjecture on the non-zeroness of the coefficients of Ramanujan's tau function.