Real-world applications of mathematics, by arxiv subject area? - MathOverflow

Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:38:56Z

What are the most important applications outside of mathematics of each of the major fields of mathematics? For concreteness, let's divide up mathematics according to arxiv mathematics categories, e.g. math.AT, math.QA, math.CO, etc.

This is a community-wiki question, so please edit and improve pre-existing answers: let's keep it to a single answer for each subject area.

(This is inspired by Terry Tao's recent post about a periodic table of the elements listing commercial applications. He suggested it might be fun to have such a summary for either the MSC top-level subjects or the arxiv subjects.)

I'd like to propose that for areas in which the applications are either numerous, non-obvious, or generally worthy of discussion, someone volunteers to open up a new question specifically about that subject area, and takes care of providing a summary here of the best answers produced there.

Answer by Scott Morrison for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:47:15Z

math.MG Metric Geometry

Discrete sphere packing solutions lead to error-correcting codes.
The earthmover metric is used in image recognition and classification.
Triangulation using the Euclidean metric is used for navigation (and nowadays, in GPS systems)

The Banach fixed point theorem for contraction mappings has a beautiful application in image compression, called fractal compression. One starts with a complete metric space $X$ of images with Hausdorff metric. Then for a given image $x \in X$ one finds a contraction mapping $A: X \to X$ with (unique) fixed point $x$. To do this, one considers self-similarities in the picture (that's why it is called fractal compression).

Then we get rid of the original image and store the map $A$ only. To reconstruct the image, one starts with any $x_0 \in X$ (for example an image which is all black or all white), and applies $A$ several times. The result will be close to $x$.

When I (Evgeny Shinder) first learnt this in high school (my friend and I implemented fractal compression as a final project for a programming class), I was fascinated how such abstract math can be applied to such a concrete problem as image compression!

Answer by Mikael Vejdemo-Johansson for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:53:31Z

math.AT Algebraic Topology

Algebraic Topology finds applications in sensor network design, coverage analysis for sensor networks, and in expanding data analysis techniques to give better visualizations for large data sets.
It has also been applied to computer vision and pattern recognition algorithms, for instance here.
Algebraic Topology can be used to design roboter arms, in particular to see which points in space it can reach given so and so many joints with such and such an angle of flexibility...
Knot theory is used when dealing with protein folding and other analysis of DNA function. There are enzymes called 'topoisomerases' that change the knottedness of loops of DNA. In fact, when bacteria (which have circular 'chromosomes' called plasmids) reproduce, they make use of an enzyme whose specific role to to unlink Hopf links! There are antibiotics that target this enzyme.
Model categories have been used in the study of concurrency. See this paper by Gaucher.

Answer by Mikael Vejdemo-Johansson for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:55:11Z

math.CT Category Theory

Category Theory helps design modern and novel programming languages, that end up being able to do optimizations based on mathematical theorems, and even allow provably correct code with less effort than using other techniques. For example, the language Haskell makes use of many ideas from category theory.
There are deep connections between category theory and logic, in the sense of computer science.
Feynman diagrams form a (monoidal) category.

Answer by Mikael Vejdemo-Johansson for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:57:02Z

math.GR Group Theory

Group theory provides methods for understanding the Rubik's cube, and for generating algorithms for solving the cube remarkably quickly from any state the cube may be in.
Groups find various applications in chemistry, eg. in the study of crystal structures and spectroscopy.
Cryptography - various hard algorithmic problems about groups are used to design crypto-systems.
Groups of symmetries are used to reduce the dimension of parameter spaces in engineering models to make model verification more tractable.

Answer by Mikael Vejdemo-Johansson for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:58:47Z

math.NT Number Theory

From Number Theory comes the ideas and theoretical basis for modern cryptography, used to secure communications everywhere from banking to cellphones.
A more quirky one is SETI (the primes in binary would be a very clear indication of a signal non-natural origin, and would be a starting point for communication).
There is a gamma ray telescope design using mod p quadratic residues to construct a mask. Gamma rays cannot be focused, so this design uses a redundant array of detectors separated from the mask to reconstruct directional information.

Answer by Qiaochu Yuan for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T04:59:23Z

math.CO Combinatorics

Combinatorics finds applications in computer science, especially in the run-time analysis of algorithms. It has also in recent years found applications in physics, at least in part via its relationship to quantum theory.
Combinatorial group testing allows one to quickly isolated defects in a large collection of samples by testing batches of samples at a time.
Combinatorial designs are routinely used to design experiments in applied statistics and quality control.
Combinatorial optimization in Logistics and Operations research.
Graphs are used as models for networks (e.g., internet server connections). Applications include searches for least expensive plane tickets, optimizing garbage pickup routes.
Graph models are used in machine learning.
combinatorial properties of permutation group was the fundamental part of Enigma cipher breaking by Marian Rejewski before II-nd WW. It is dated now but it was one of the first application of higher mathematic reasoning to cipher breaking, because before usually linguistic reasoning was used instead.

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T05:12:37Z

math.ST Statistics

Accurate polling
Fraud detection (e.g. via Benford's law)
Used in the world of finance, economics and gambling on a daily basis.
Predicting consumer preferences (e.g. Netflix prize)
Experimental design and hypothesis testing (e.g. testing of medical hypotheses)
Machine learning
Quality control

Answer by Steven Sam for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T05:40:07Z

math.CV Complex Variables

Conformal mapping simplifies various problems about heat conduction and fluid flow, such as calculating steady temperatures. Modelling the flow of fluids around solid bodies (for example aircraft wings!) can also be simplified by appropriate conformal mappings.
Tools from complex analysis (phasors, argument principle, conformal mapping) are widely used in analyzing electrical circuits, and stress and strain analysis in mechanical engineering.
The uniformization theorem for discrete complex analysis (circle packings) allows for efficient and pleasant visualization of geometric data that can be related to surface triangulations.

Answer by Harrison Brown for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T05:56:32Z

math.RT Representation Theory

Much of modern particle physics is related to representations of Lie algebras. For instance, Gell-Mann's "Eightfold Way" comes from the representation theory of SU(3) and its associated algebra.
Almost every application of theoretic physics in solid state physics extensively uses representation theory in description of periodic and quasi-periodic media as crystals, semiconductors etc. In fact solutions of Schroedinger Equation in such cases, numerical or analytical has to be carried in accordance with representation of crystal/quasi cristal symmetry group.

Answer by Scott Morrison for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T06:01:52Z

math.RA Rings and Algebras

Google's Pagerank algorithm is based, in part, on the singular value decomposition.
Fourier analysis / transforms and linear algebra is at work in the world millions of times per second (video, audio). In particular, creating or displaying a JPEG image requires the discrete fourier transform.
Quaternions are used in 3D modeling and animation software to represent rotations in a more robust form than Euler angles (helping to avoid transition issues like gimbal lock).
Every simulation using Finite Element Method uses algebra in very extensive way.

Answer by Javier Moreno for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T06:50:42Z

math.LO Logic

Lambda calculus, the theoretical basis for functional programming (Lisp in particular), was developed by Alonzo Church in the 1930's as part of his research on recursion and the foundations of mathematics.
Formal verification is used to verify software and hardware in which failure rates need to be as close to zero as possible (e.g. avionics)
Finite Model Theory is used to design and improve database query systems.
logical reasoning is widely and sometime non-trivial used in law application in courts, and in should be in parliaments during setting a law systems

Answer by Proportional for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T10:18:35Z

math.OC Optimization and Control

Optimization is heavily used in medicine for cancer treatment, in a technique of radiation therapy called Intensity Modulated Radiotherapy (IMRT) Take a look at this presentation of one of the main researchers in the field.
Used everywhere in almost every electro-mechanical system (cars, planes, the power grid)
Kalman filtering and the like are used to enable (e.g.) radar tracking.
Optimization (and more generally, operations research) is used for management in logistics and analyses/feasibility studies.

Answer by Thomas Sauvaget for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T12:19:55Z

Math.AP Analysis of PDE

Partial differential equations are used a lot for modelling systems in biology and medicine, and help describe e.g. animal coat pattern formation (zebras, leopards...), wound healing, tumor growth, spread of a virus in a population, predator-prey systems in ecology, predicting the variations of concentrations of chemicals (hormones, drugs...) within an organ over time...
PDEs are used in climate modelling, from atmospheric dynamics to ocean currents.
Radar imaging is based on solving an inverse problem. The recent buzz about metamaterials and invisibility is based on understanding variable-coefficient elliptic problems.

Answer by Graham Jones for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T12:50:43Z

math.PR Probability

Stochastic processes such as branching processes and HMMs are used to model speciation and extinction (the Tree of Life), evolution of molecular sequences, cell proliferation, and other things in biology. For example, see `Branching Processes in Biology' by Kimmel and Axelrod.
Here's a somewhat frivolous one (but one that casinos greatly care about): the number of times one needs to shuffle a deck before it truly randomizes.
used in the world of finance, economics and gambling on a daily basis.
Random number generation is a key component of many efficient algorithms, and also plays an important role in cryptography.
Stochastic calculus is used to price options (Black-Scholes formula) and to hedge against risk. (Of course, it is not always applied wisely...)

Answer by Anton Geraschenko for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T15:20:26Z

math.IT Information theory

Compression; efficient use of bandwidth
Error-correcting codes protect against digital data corruption from noise, packet loss, physical damage, etc.
Used in machine learning

Answer by Anton Geraschenko for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T15:35:58Z

math.CA Classical Analysis

Fourier analysis allows one to precisely divide up the electromagnetic spectrum, leading of course to radio, television, wireless, and so forth.
The fast Fourier transform (and relatives, such as the fast Wavelet transform) is an essential component of many signal processing algorithms.
MRI is based on inverting the Radon transform.
Wavelets are used in signal processing (e.g. image compression, edge detection).

The picture is of a Hilbert spectrometer.

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T15:59:49Z

math.DG Differential geometry

Lie groups are used in robotics (to find the most efficient way to maneuver a robotic arm, for instance).
Spherical trigonometry is essential for navigation (a few centuries ago, this was THE application of mathematics to the real world - naval empires were built upon this!)
Finsler geometry can be used in planning shipping routes when ocean currents and winds (as well as the earth's curvature) need to be taken into account to conserve fuel.
Differential geometry (Riemann metrics + stress tensors) is used in mechanical engineering to study the properties of large membranes, for example, how one should go about building a large tent. Keyword for literature search "elastic membrane", "continuum mechanics".
Without taking into account the effects of general relativity on the orbiting satellites that make up the GPS system, the locations reported by GPS receivers would accumulate errors of around 10km each day, rendering the system useless.
Nonlinear control theory makes heavy use of differential geometry

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T16:13:22Z

math.AG Algebraic geometry

Elliptic curve cryptography
Configuration spaces of robot arms are often semi-algebraic sets, and algebraic geometry has been used to understand their geometry.
Algebraic geometry over finite fields is used to construct error correcting codes.
Statistical models are often semi-algebraic sets, and algebraic geometry can be used to devise tests for the correctness of the model or to fit parameters.

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T16:17:59Z

math.SP Spectral theory

Spectroscopy (of course)
To avoid bridge collapse, knowing where the resonant frequencies are is extremely important. :-)

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T16:46:19Z

math.AC Commutative algebra

Reed-Solomon codes (a type of error correction codes based on polynomials over finite fields - this is why CDs and DVDs still work even after being scratched!)

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T17:41:06Z

math.GM General Mathematics

The cosmic distance ladder is largely built using elementary geometry (although for some legs of the ladder, more advanced mathematics, e.g. relativity and probability, play a role).
Mathematics considered as general is the model of precise reasoning, which is widely ( not so widely as it could be or even should be probably...) used in various branches of everyday living as law, economics, psychology, rhetoric etc. Person who do not know mathematics at all usually have small chances to solve his problems in abstract way, which means that every problem may only certain solution related to defined example, without generalisation.

Answer by Terry Tao for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T20:11:27Z

math.NA Numerical analysis

Linear programming algorithms are used in compressed sensing, which is now being used in MRI and imaging to increase resolution and/or decrease the number of measurements required.
Numerical analysis is what makes calculators work. (And so much more!)

Answer by Anand Sarwate for Real-world applications of mathematics, by arxiv subject area?

2009-10-26T23:45:55Z

math.FA Functional Analysis

Used in signal processing for modeling and design.
Used in machine learning in the design of classifiers (e.g. spam filters)

Answer by Ari for Real-world applications of mathematics, by arxiv subject area?

2009-10-27T00:02:27Z

math.SG Symplectic Geometry

Symplectic integrators are used for numerical simulation of Hamiltonian mechanics. Prominent applications include molecular dynamics, solar system dynamics, computer animation, and a wide variety of problems in mechanical engineering.

Answer by Orr Shalit for Real-world applications of mathematics, by arxiv subject area?

2009-10-27T03:33:36Z

math.OA Operator Algebras

(In "Nature") Operator algebras, and, more broadly, operator theory, appear in mathematical models for quantum phenomena.
(In Engineering) Completely positive maps are used in quantum information theory. There are also many connections between operator algebras and wavelets, which is useful in electrical engineering.

Answer by Anna Varvak for Real-world applications of mathematics, by arxiv subject area?

2009-10-28T11:23:06Z

Math.HO: History and overview: used for

understanding mathematics as a social and human endeavor
considering alternative approaches that historically have been used to explore quantifiable relationships
to learn us that no mathematical ideas appear in empty social space: every idea its a child of its time. It is application of HO to mathematics itself

Answer by Anna Varvak for Real-world applications of mathematics, by arxiv subject area?

2009-10-28T11:30:05Z

Math.DS: Dynamical systems

Modeling flow of liquids, like the animated flow of lava in the movie "Volcano"
Solving classical few-body problems, though it doesn't help with the modern notorious "two-body problem"
Heteroclinic trajectories are used in space mission design
Heteroclinic tangles are used by chemical engineers to get well-mixed reactants

Answer by Ken Graham for Real-world applications of mathematics, by arxiv subject area?

2010-02-10T15:33:09Z

math.GR Group Theory: used for

Card tricks that don't work by sleight of hand, but via the arrangements of the cards. e.g. Sim Sala Bim, see this site for description and a graphical explanation. If you think about it, the symmetric group explains the trick and shows you how you to extend it past three piles of seven cards, but to N piles of M cards.

Answer by Andy for Real-world applications of mathematics, by arxiv subject area?

2010-03-11T20:47:04Z

math.PR Probability

Markov chains are used to find uniformly random objects. This, among other things, makes designing an experiment fairest and crypto-systems based on designs securest.