## Where have you used computer programming in your career as an (applied/pure) mathematician?

For background: I'm working on a book to help mathematicians learn how to program. However, I need to see some examples from people in the field that have done different kinds of things than I have.

Where have you used programming in your career as a mathematician? (If you haven't feel free to say so, though it isn't very helpful)

I've currently used programming in several math-y settings. Computational Biology, Image Processing (Fourier Transforms and other things like that), writing scripts that comply to a certain data restriction or to a library. I've looked at some computational algebraic geometry, but not much as of yet, and I'd use SAGE or sympy if I needed that.

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This probably should be community wiki. – Andy Putman May 9 2010 at 3:28
I've wiki-hammered this. – Scott Morrison May 9 2010 at 4:11
oh, sorry about that, I haven't posted in a while and forgot to hit community – Michael Hoffman May 9 2010 at 8:04
You may also want to look at the recent question on experimental mathematics. Some answers there may apply: mathoverflow.net/questions/12085/… – Willie Wong May 9 2010 at 11:01
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I've used computers to do some group theory calculations. I was too lazy to figure out how to do them in GAP, so I wrote the programs I needed in C++. One particular paper that I used them in is my paper "The Picard Group of the Moduli Space of Curves with Level Structures". The paper and code are available on my webpage at http://www-math.mit.edu/~andyp/papers/PicardGroupLevel.html

In that paper, I needed to know some twisted group cohomology groups. The groups depended on a parameter $g$, but I could prove that they were independent of the parameter once $g$ was large, so I was able to reduce myself to computing a finite number of cases on a computer.

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I've used programming in MATLAB for countless things. Some highlights:

• bioinformatics: besides testing published algorithms, I also developed algorithms for producing surrogate *NAs with completely specified short-range subsequences and biologically plausible codon structure, for molecular phylogeny, and a toy model of *NAs with specified nearest-neighbor thermodynamical properties. In related work I used a numerical calculation to rule out a class of hypotheses about the binding kinetics of oligomers;
• networks: modeling queueing networks and prototyping network monitoring data structures, visualizations and algorithms involving generalized statistical physics and continuous-time martingales and change detection techniques, as well as post-processing outputs from multiple prototype network monitoring systems. Besides prototyping, I've also used MATLAB for QA purposes in my company;
• I outlined some combinatorial calculations about necklaces in MATLAB and C for porting to a reconfigurable computer;
• I elucidated the structure of the Lie algebra of the stochastic group, particularly completely explicit Levi decompositions (this is no trivial feat in a numerical language)--someday I'll clean this up and put it on the arXiv;
• I produced periodic lattices with permutohedral boundary conditions (used so far to validate a 2D lattice Boltzmann model by approximating the initial decay of a Taylor-Green vortex);
• I enumerated the minimal periodic colorings of the root lattice $A_N$ for $N$ small by means of permutation matrices;
• I've analyzed detailed behavior of Anosov systems (e.g., the cat map and a map topologically conjugate to the cross section of the geodesic flow on a surface of negative curvature).

I'm sure I could think of other stuff that has been especially useful to my work. I use MATLAB more days than not.

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 Hoping you'll get those Levi decompositions on arxiv someday, I'm looking at nonnumerical computations in particular. – Michael Hoffman May 17 2010 at 3:38 If you want I can send you a rough draft. My email is sh [at my domain name]. – Steve Huntsman May 17 2010 at 3:42

Look at any of my published papers! :-)

I've

1. Solved huge systems of quadratics in order to construct exotic subfactors.
2. Calculated R-matrices for representations of various quantum groups, in order to systematically identify coincidences amongst small modular tensor categories.
3. Implemented a parallelisable, caching algorithm for computing Khovanov homology, in an attempt to disprove the smooth 4-d Poincare conjecture.
4. Enumerated bipartite graphs satisfying certain combinatorial conditions, filtered by largest eigenvalue, in order to classify subfactors up to index 5 (slides).
5. Looked for small real cyclotomic integers which are larger than all their conjugates, in order to verify the many cases of a theorem identifying those smaller than 76/33.
6. Proved identities involving q-binomial coefficients using the methods of A=B.

and then a whole lot of stuff that hasn't yet, or won't ever, make it into print.

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 Thanks Scott, I really appreciate it! I'll read them ASAP – Michael Hoffman May 9 2010 at 8:05 Well, sometimes the computer code is slightly below the surface. But feel free to ask if you want more details. – Scott Morrison♦ May 9 2010 at 16:43

I've worked (and still continue to work) on K-nearest-neighbor analysis for image segmentation and multispectral and hyperspectral data classification, and worked on trying to improve the algorithms for speed and to require less interaction. I've also worked on writing C programs, which work much faster than interpreted Octave or Matlab code, for image analysis and texture analysis, and for image segmentation based on local feature analysis. This is all very different from the types of mathematics I used and learned in academia.

I've also run simulations of some games I've played recreationally to answer some simple questions using backtracking algorithms, for example, a card game akin to playing tic-tac-toe on a 4-dimensional lattice which I used as an answer to Favorite mathematical puzzles and toys.

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I've used computers to prove a theorem about a complex dynamical system. See http://www.ams.org/mathscinet-getitem?mr=1836429. So have others. See Proving Conjectures by Use of Interval Arithmetic.

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I often write small programs in various languages (Perl, C, Maple, HP 50G) to generate complicated pictures, perform tedious algebraic computations to test conjectures, or simulate random processes.

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I am a pure mathematician interested in representation theory.

I computed $q$-characters of $\ell$-fundamental representations for the quantum affine $E_8$ by a SUPERCOMPUTER. See

http://arxiv.org/abs/math/0606637

There is more famous project on $E_8$:

http://www.aimath.org/E8/

I believe there are lots of other computation in the representation theory of exceptional groups, which require lots of memory.

They are usually based on recursive algorithms, and one cannot use the parallel computing.

When I computed $q$-characters, I could not find any guides explaining how to code a program for such a problem. I appreciate very much if an expert could give me any references.

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I wrote a series of algorithms to find bounds on the homology of finitely-presented groups and implemented them in GAP in http://www.intlpress.com/HHA/v12/n1/a3/. Graham Ellis has written the GAP package HAP, http://www.gap-system.org/Packages/hap.html, which does some group (co)homology calculations.

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