I’m the Mathematics Director for WisCEL and a member of the mathematics department of the University of Wisconsin. I study combinatorics, specifically the combinatorics of orthogonal polynomials. I was previously at the University of Puget Sound, KAIST, and the University of Minnesota. Here’s how to contact me.
email:  ddr...@math.wisc.edu. I digitally sign my messages with GPG; my key is here (key id 58C2E4BA). The digital signature is the little text attachment you see on emails from me. 
office:  Van Vleck Hall 407; I'm also frequently in the WisCEL space in College Library 3260. 
phone: 
608.263.4918 in Van Vleck; 608.890.4489 in College Library.

snail mail: 
Department of Mathematics
480 Lincoln Dr
Madison, WI 53706

Spring semester 2013, I'm teaching Math 112, section 13, and Math 101 in the College Library WisCEL space.
I finished my PhD at the University of Minnesota in 2006. Here’s my thesis, and here’s the known errata list.
At FPSAC 2007 I gave a talk on the combinatorics of associated Hermite polynomials. I have slides from the talk, and a preprint of the paper is available at arXiv:0709.0987. The paper appeared in the European Journal of Combinatorics: doi:10.1016/j.ejc.2008.05.009 (here is citation info).
Jang Soo Kim and I have a preprint: kdistant crossings of matchings and set partitions, which was accepted as a poster at FPSAC 2009 and appears in the proceedings.
See arXiv:0909.1655. This paper was published in Advances in Applied Mathematics: doi:10.1016/j.aam.2009.12.008. I gave a talk on this paper at the 2012 Combinatorial Potlatch in Vancouver; here are the slides.
See arXiv:1006.1959. In the kdistant noncrossing matchings paper above, Jang Soo and I proved that little Schroeder paths are equal to the generating function for certain weighted Dyck paths; this paper presents a bijective proof of that fact and examines its consequences. This appeared in the Journal of Integer Sequences. I gave a talk on this bijection at the Korea Combinatorics Workshop at Yeungnam University; here are the slides. I also gave a talk on the bijection at the fall 2010 KMS meeting at Postech: here are those slides which are an improved version of the Yeungnam ones. The Sage demo I used is here.
I talked about this bijection at the KPP combinatorics seminar, in the larger context of the combinatorics of Chebyshev, Hermite, and Charlier orthogonal polynomials. Here are the slides.
This is joint work with Ryan Gantner and appears in the Journal of the Chungcheong Mathematical Society; see their page or arXiv:1109.3273.
KarlDieter Crisman and I organized an MAA PREP workshop on Sage during the summer of 2012.
I’m the main author of the SageTeX package, which allows you to easily pull the results of Sage computations and plots into your LaTeX document. And since Sage is based on Python, you can write Python that writes LaTeX for you. This is really useful! SageTeX is a (admittedly small) component of an NSF grant for “integrating open mathematics software and open educational materials into the mathematics curriculum and classroom”.
Here’s the documentation and the typeset example file.
Try it today. If you’re really curious, you can follow SageTeX development at bitbucket.
I maintain the KAIST Sage server(s). If you are interested in an account on the Memorial Day server, please email me. Otherwise, anyone can use the Groundhog Day server. (Read about the difference between the two servers.)
In 2007, I developed some Java applets that demonstrate the geometric operations of truncation, expansion, and snubification on the Platonic solids. My goal was to get all of the Archimedean solids this way; it turns out to be impossible, but I enjoyed making the applets and you may find them interesting.
That page also includes a little miniessay on the freedoms and restrictions that mathematical software affords and imposes on you.
GeoGebra is a Java program with which you can easily make very cool interactive geometry demonstrations. Its focus is on elementary Euclidean geometry, but I’ve discovered that you can use it for an impressive variety of 2dimensional graphics. It’s also extremely easy to make web pages with GeoGebra – see my pages on the complex cosine or polar plotting or Riemann sums for definite integrals. Try doubleclicking on the applet to open the full GeoGebra program!
I’ve also published a (IMHO) very nice applet demonstrating the epsilondelta definition of limits.
TikZ is a graphics system for TeX and friends. It’s very powerful, and somewhat similar to PSTricks. I highly recommend TikZ (and its lowerlevel engine, PGF) for all your graphics needs when writing LaTeX documents. Here are some examples of things I’ve made with TikZ:
Here are some old notes and other things I wrote back in grad school; according to server logs, they’re relatively popular and I should update and improve these sometime, but for now, here they are:
I use and contribute to Sage, an opensource mathematics computing system whose mission is to create a viable free open source alternative to Magma, Maple, Mathematica and Matlab. Read about why you should use Sage and download it today!
I’ve set up a public Sage server here at KAIST: you can access it at http://sagenb.kaist.ac.kr
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Zotero is a web service that is a great tool for helping research. It keeps track of your references, makes it super easy to capture information, search and sort that information, and then put references into your documents. It has synchronization features that make it super easy to have access to all your data everywhere you go.
Zotero works mostly as a Firefox plugin, but is now available as a standalone program. When it was only a Firefox plugin, I initially thought putting a citation manager into your browser was not a great idea, but think about it: if you are using a computer to look up information about a reference, what kind of software are you using? You’re using a web browser – so put the citation manager where the action is. Zotero is open source and works very well. I recommend it.
CiteULike is a great service for keeping track of all your references. It stores PDFs, lets you add notes, and is moving towards some useful social networking features. Have a look at the things I’ve bookmarked. I’ve mostly abandoned CiteULike in favor of Zotero, but it’s still a nice service.
I’ve used Linux in some form or another so long that it’s surprising my beard is so short. I recommend installing Ubuntu (or another Linux distribution). It works really well, doesn’t crash, and is easy to install. Get it today!
I was one of the organizers of the inaugural Graduate Student Combinatorics Conference at the University of Minnesota in April 2005. I’m delighted to see that the conference is continuing, and even getting NSF funding!