Occurrences of (co)homology in other disciplines and/or nature - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-19T15:56:33Z http://mathoverflow.net/feeds/question/60108 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature Occurrences of (co)homology in other disciplines and/or nature Noah Giansiracusa 2011-03-30T19:15:19Z 2012-10-25T06:34:18Z <p>I am curious if the setup for (co)homology theory appears outside the realm of pure mathematics. The idea of a family of groups linked by a series of arrows such that the composition of consecutive arrows is zero seems like a fairly general notion, but I have not come across it in fields like biology, economics, etc. Are there examples of non-trivial (co)homology appearing outside of pure mathematics? </p> <p>I think Hatcher has a couple illustrations of homology in his textbook involving electric circuits. This is the type of thing I'm looking for, but it still feels like topology since it is about closed loops. Since the relation $d^2=0$ seems so simple to state, I would imagine this setup to be ubiquitous. Is it? And if not, why is it so special to topology and related fields?</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/60116#60116 Answer by Chris Gerig for Occurrences of (co)homology in other disciplines and/or nature Chris Gerig 2011-03-30T20:03:04Z 2011-03-30T20:19:29Z <p>Robert Ghrist is all about applied topology: Sensor Network, Signal Processing, and Fluid Dynamics. (homepage: <a href="http://www.math.upenn.edu/~ghrist/index.html" rel="nofollow">http://www.math.upenn.edu/~ghrist/index.html</a> ). For instance, we want to use the least number of sensors to cover a certain area, such that when we remove one sensor, a part of that area is undetectable. We can form a complex of these sensors and hence its nerve, and use homology to determine whether there are any gaps in the sensor-collection. I've met with him in person and he expressed confidence that this is going to be a big thing of the future.</p> <p>There are also applications of cohomology to Crystallography (see Howard Hiller) and Quasicrystals in physics (see Benji Fisher and David Rabson). In particular, it uses cohomology in connection with Fourier space to reformulate the language of quasicrystals/physics in terms of cohomology... Extinctions in x-ray diffraction patterns and degeneracy of electronic levels are interpreted as physical manifestations of nonzero homology classes.</p> <p>Another application is on fermion lattices (http://arxiv.org/abs/0804.0174v2), using homology combinatorially. We want to see how fermions can align themselves in a lattice, noting that by the Pauli Exclusion principle we cannot put a bunch of fermions next to each other. Homology is defined on the patterns of fermion-distributions.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/60124#60124 Answer by José Figueroa-O'Farrill for Occurrences of (co)homology in other disciplines and/or nature José Figueroa-O'Farrill 2011-03-30T20:57:47Z 2011-03-31T00:06:54Z <p>Quantum field theory is outside the realm of pure mathematics, makes contact with the real world and features chain complexes and cohomology.</p> <p>The current paradigm for gauge theories such as the standard model is based on Yang-Mills theories coupled to matter. The quantisation of nonabelian (and, depending on your choice of gauge fixing function, also abelian) Yang-Mills theories features a cohomology theory known by the moniker of BRST, after the inventors: Becchi, Rouet, Stora and, independently, Tyutin. The cleanest proofs of the renormalizability of Yang-Mills theories are cohomological in nature.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/64176#64176 Answer by Pasha Zusmanovich for Occurrences of (co)homology in other disciplines and/or nature Pasha Zusmanovich 2011-05-07T08:43:48Z 2011-05-07T08:43:48Z <ol> <li><p>Something resembling de Rham complex with differential-algebraic flavor appears in (variant of) control theory, see, for example, G. Conte, C.H. Moog, A.M. Perdon, Algebraic Methods for Nonlinear Control Systems, 2nd ed., Springer, 2006. But, as far as I can tell, they do not use the world "cohomology" explicitly.</p></li> <li><p>Spencer cohomology (which is, essentially, a Lie algebra cohomology) appears as obstructions to integrability of some differential-geometric structures (G-structures) and, through it, of (some) differential equations. Potentially this opens a wide possibilities for applications, and indeed, Dimitry Leites advocates this approach in (some of) his writings. An emblematic publication which is available, unfortunately, only in Russian, is: "Application of cohomology of Lie algebras in national economy", Seminar "Globus", Independent Univ. of Moscow, Vol. 2, 2005, 82-102. The Russian original for "national economy" in the title is (a somewhat pejorative and untranslatable term) "narodnoe khozyai'stvo".</p></li> </ol> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/97476#97476 Answer by Xiao-Gang Wen for Occurrences of (co)homology in other disciplines and/or nature Xiao-Gang Wen 2012-05-20T13:30:55Z 2012-05-30T23:24:29Z <p>Recently, it is realized that quantum many-body states can be divided into short-range entangled states and long-range entangled states.</p> <p>The quantum phases with long-range entanglements correspond to <a href="http://en.wikipedia.org/wiki/Topological_order" rel="nofollow">topologically ordered phases</a>, which, in two spatial dimensions, can be described by tensor category theory (see <a href="http://arxiv.org/abs/cond-mat/0404617" rel="nofollow">cond-mat/0404617</a>). Topological order in higher dimensions may need higher category to describe them.</p> <p>One can also show that the quantum phases with short-range entanglements and symmetry $G$ in any dimensions can be <a href="http://arxiv.org/abs/1106.4772" rel="nofollow">"classified"</a> by Borel group cohomology theory of the symmetry group. (Those phases are called symmetry protected trivial (SPT) phases.)</p> <p>The quantum phases with short-range entanglements that break the symmetry are the familar Landau symmetry breaking states, which can be described by group theory.</p> <p>So, to understand the symmetry breaking states, physicists have already been forced to learn group theory. It looks like to understand patterns of many-body entanglements that correspond to topological order and SPT order, physicists will be forced to learn tensor category theory and group cohomology theory. In modern quantum many-body physics and in modern condensed matter physics, tensor category theory and group cohomology theory will be as useful as group theory. The days when physics students need to learn tensor category theory and group cohomology theory are coming, may be soon.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/97480#97480 Answer by Lee Mosher for Occurrences of (co)homology in other disciplines and/or nature Lee Mosher 2012-05-20T14:10:19Z 2012-05-20T14:10:19Z <p>My understanding, from conversations with Raoul Bott, is that his early work on <a href="http://books.google.com/books?id=j2vFhxA5K-UC&amp;pg=PA75&amp;lpg=PA75&amp;dq=Bott+Duffin+theorem&amp;source=bl&amp;ots=8bgmTfSJO0&amp;sig=2hA0ApbayDfVcemzGqMGqSxFXtI&amp;hl=en&amp;sa=X&amp;ei=Q_m4T4OIJMmZ6QH5g83NCw&amp;ved=0CE0Q6AEwBA#v=onepage&amp;q=Bott%2520Duffin%2520theorem&amp;f=false" rel="nofollow">electrical circuits and the Bott-Duffin theorem</a> can be intepreted as exhibiting close connections between de Rham cohomology and the laws of electrical circuits, and that this is part of what led him into pure mathematics early in his career.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/97481#97481 Answer by John Klein for Occurrences of (co)homology in other disciplines and/or nature John Klein 2012-05-20T14:18:37Z 2012-05-20T14:18:37Z <p>A classical and elegant application is to the solution of Kirchhoff's theorem on electrical cricuits. See:</p> <p>Nerode, A.; Shank, H.: An algebraic proof of Kirchhoff's network theorem. <em>Amer. Math. Monthly,</em> <strong>68</strong> (1961) 244–247</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/97482#97482 Answer by Adam Azzam for Occurrences of (co)homology in other disciplines and/or nature Adam Azzam 2012-05-20T14:44:52Z 2012-05-20T14:44:52Z <p>It's my understanding that Carina Curto and Vladmir Itskov at the University of Nebraska - Lincoln apply algebraic topology (among other things) to study theoretical and applied neuroscience. </p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/97672#97672 Answer by Francois Ziegler for Occurrences of (co)homology in other disciplines and/or nature Francois Ziegler 2012-05-22T15:07:51Z 2012-05-23T01:59:27Z <p>The <em>mass</em> of a classical mechanical system is an element in the (one-dimensional) second cohomology group of the Lie algebra of the Galilei group. See J. M. Souriau, <a href="http://www.jmsouriau.com/structure_des_systemes_dynamiques.htm" rel="nofollow">Stucture des Systèmes Dynamiques</a>, Chap. III, section (12.136). Or in english translation, search inside <a href="http://www.amazon.com/dp/0817636951/" rel="nofollow">here</a> for "total mass".</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/98167#98167 Answer by Yi-Zhi Huang for Occurrences of (co)homology in other disciplines and/or nature Yi-Zhi Huang 2012-05-28T05:15:48Z 2012-05-28T05:15:48Z <p>Cohomology is basically a way to get information from linear maps that are neither injective nor surjective. So in any place such linear map occurs, one can find the use of cohomology. But whether cohomology plays a significant role will depend on what kind of questions asked. </p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/98168#98168 Answer by Yi-Zhi Huang for Occurrences of (co)homology in other disciplines and/or nature Yi-Zhi Huang 2012-05-28T05:30:39Z 2012-05-28T05:30:39Z <p>Every cohomology group has its meaning but the meaning of higher cohomologies is still to be explored. Physics and applications in other discipline of science might provide motivation and inspiration for the study of such meanings. In algebra, the first cohomology usually corresponds to some kind of homomorphism type property, for example, derivations and so on. The second cohomology corresponds what people call square-zero extensions of the algebra and also infitesimal deformations of the algebra. The thrid cohomology of associative algebras is the obstruction to the existence of finite formal deformations. If one sees such extensions of algebras or deformations of algebras in problems in other sciences, then it is the cohomology that plays the fundamental role here. </p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/98179#98179 Answer by none for Occurrences of (co)homology in other disciplines and/or nature none 2012-05-28T08:12:18Z 2012-05-28T08:12:18Z <ul> <li>There's a CST.SE thread of possible interest here: <a href="http://cstheory.stackexchange.com/questions/7958/papers-on-relation-between-computational-complexity-and-algebraic-geometry-topol" rel="nofollow">http://cstheory.stackexchange.com/questions/7958/papers-on-relation-between-computational-complexity-and-algebraic-geometry-topol</a></li> </ul> <p>It mentions stuff like Geometric Complexity Theory, a far-out program for proving P!=NP with algebraic geometry.</p> <ul> <li>Similarly here: <a href="http://cstheory.stackexchange.com/questions/2898/applications-of-topology-to-computer-science" rel="nofollow">http://cstheory.stackexchange.com/questions/2898/applications-of-topology-to-computer-science</a></li> </ul> <p>Mentions the thing I actually first websearched for, Herlihy's work on concurrent and distributed computing using cohomology.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/99212#99212 Answer by Patricia Hersh for Occurrences of (co)homology in other disciplines and/or nature Patricia Hersh 2012-06-10T02:11:10Z 2012-10-24T22:48:20Z <p>Anders Björner and László Lovász used bounds on the Betti numbers for the complement of a real subspace arrangement called the $k$-equal arrangement to give a complexity theory lower bound that agreed, up to a scalar multiple, with the previously known upper bound in:</p> <p>A. Björner and L. Lovász, Linear decision trees, subspace arrangements, and Mobius functions, Journal of the American Mathematical Society, Vol. 7, No. 3 (1994), 677--706.</p> <p>The basic question addressed in their paper (along with other questions of a similar flavor) is how many pairwise comparisons of coordinates are needed to decide if a vector in ${\bf R}^n$ has $k$ coordinates all equal to each other for fixed $k$ and $n$. They observed that this is equivalent to deciding whether the vector lies on the so-called $k$-equal arrangement or in its complement, where the $k$-equal arrangement is the subspace arrangement comprised of the ${n\choose k}$ subspaces where $k$ coordinates are set equal to each other. </p> <p>To this end, they gave a lower bound on the number of leaves in a linear decision tree -- a tree where one starts at the root, and each time one does a comparison of two coordinates $a_i$ and $a_j$, then one proceeds down to either the $a_i &lt; a_j$ child or the $a_i = a_j$ child or the $a_i > a_j$ child. One reaches a leaf when no further queries are necessary to make a decision as to containment in the arrangement or its complement. The log base 3 of the number of leaves is a lower bound on the depth of the tree, i.e. on the number of queries needed in the worst case.</p> <p>To get some intuition for why this bound depended fundamentally on the Betti numbers of the complement, consider the $k=2$ case -- where the number of connected components of the complement of the subspace arrangement (which in this case is a hyperplane arrangement) is an obvious lower bound on the number of leaves in any linear decision tree. </p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/99237#99237 Answer by Alexander Chervov for Occurrences of (co)homology in other disciplines and/or nature Alexander Chervov 2012-06-10T12:44:48Z 2012-06-10T12:50:31Z <p>Actually even schoolchildren calculate group co-cycle. (Without knowing that it is called like this). Cohomology occurs in everyday life as soon as one learns to count. </p> <p>5+7 = <strong>1</strong> 2 </p> <p>4 + 5 = <strong>0</strong> 9</p> <p>2 + 8 = <strong>1</strong> 0 </p> <p>What is the function on which sends a pair (a,b) to the $0$ or $1$ depending result is greater than 9 or not ? ( e.g. f(5,7)= <strong>1</strong>, f(4,5) = <strong>0</strong>, f(2,8)= <strong>1</strong>). </p> <p><strong>This is actually a 2-cocycle for group $Z/nZ$ with values in $Z$.</strong></p> <p>It can be checked directly or...</p> <p>Let us look on it more conceptually. Consider the standard short exact sequence of abelian groups $0->Z->Z->Z/n->0$. (First map is multiplication by $n$, the second is factorization and will be denoted by $p$).</p> <p>Choose section $s: Z/nZ -> Z$ (i.e. any map such $ps=Id$, where $p: Z->Z/nZ$, it is like connection in differential geometry (can be made precise)).</p> <p>Define $f(a,b)= s(a)+s(b) - s(a+b)$</p> <p>Note that: a) this function $f(a,b)$ is exactly we talked above</p> <p>b) from general theory this is 2-cocyle, (it corresponds to this extension, (it it is like "curvature" of connection is differential geomety (can be made precise)).</p> <p>That is all: we explained why it is group cocycle and what its role.</p> <hr> <p>I would like to learn this 20 years ago when I learned group cohomology as undergraduate, but I learned this 1 ago, doing some engineering work in wireless communication... I am still surprised that it is not written on the first page of any textbook which deals with group cohomology, when I am explaining this to my friends most did not know this also and after knowing share my feeling of surprise.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/99381#99381 Answer by Margaret Friedland for Occurrences of (co)homology in other disciplines and/or nature Margaret Friedland 2012-06-12T18:41:24Z 2012-06-12T18:41:24Z <p>The finite element method- a numerical method for solving PDE's- has a homological interpretation:</p> <p>MR2269741 (2007j:58002) Arnold, Douglas N.(1-MN-MA); Falk, Richard S.(1-RTG); Winther, Ragnar(N-OSLO-CMA) Finite element exterior calculus, homological techniques, and applications. (English summary) Acta Numer. 15 (2006), 1–155</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/99631#99631 Answer by Cristi Stoica for Occurrences of (co)homology in other disciplines and/or nature Cristi Stoica 2012-06-14T18:03:00Z 2012-06-14T18:03:00Z <p>An application of cohomology to provide a geometric/topological description charged particles in General Relativity can be found in</p> <p><a href="http://books.google.com/books?id=H3YZAAAAIAAJ" rel="nofollow">GRAVITATION: An Introduction to Current Research, ed. Louis Witten</a></p> <p>and references therein.</p> <p>Wheeler's geometrodynamics program contained a subprogram named "charge without charge", which aimed to express the electric charge in terms of geometric and/or topological properties. A wormhole allows the existence of an electromagnetic field without source - hence the name "charge without charge". The two ends of the wormholes behave as particles of opposite electric charge. And all this can be obtained as a solution to Einstein-Maxwell equations. Roots of the approach of Misner and Wheeler can be found in <a href="http://dieumsnh.qfb.umich.mx/archivoshistoricosmq/ModernaHist/Einstein1935b.pdf" rel="nofollow">the paper of Einstein and Rosen</a>, and a series of papers of G. Y. Rainich from 1924-1925.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/99975#99975 Answer by Cristi Stoica for Occurrences of (co)homology in other disciplines and/or nature Cristi Stoica 2012-06-19T04:58:08Z 2012-06-19T04:58:08Z <p>The <a href="http://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect" rel="nofollow">Aharonov–Bohm</a> effect. Classically, you can't distinguish two electromagnetic potentials which are in the same cohomology class. From quantum viewpoint, they can be distinguished, because an electron changes its phase under parallel transport defined by the connection associated to a potential.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/100104#100104 Answer by Mariusz for Occurrences of (co)homology in other disciplines and/or nature Mariusz 2012-06-20T09:27:34Z 2012-06-20T09:27:34Z <p>At a first glance the characterization of the structures you mention - "(co)homologies", appear to be easily interpreted as modal structures (i.e. as in modal logics), or labelled transition systems, widely used in computer science.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/100126#100126 Answer by Bruce Westbury for Occurrences of (co)homology in other disciplines and/or nature Bruce Westbury 2012-06-20T12:22:37Z 2012-06-20T12:22:37Z <p>I am surprised no-one has mentioned Persistent Homology. </p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/110598#110598 Answer by none for Occurrences of (co)homology in other disciplines and/or nature none 2012-10-24T23:55:49Z 2012-10-24T23:55:49Z <ul> <li><a href="http://sigact.org/Prizes/Godel/2004.html" rel="nofollow">http://sigact.org/Prizes/Godel/2004.html</a></li> <li><a href="http://www.cs.brown.edu/~mph/HerlihyS99/p858-herlihy.pdf" rel="nofollow">http://www.cs.brown.edu/~mph/HerlihyS99/p858-herlihy.pdf</a></li> </ul> <p>Maurice Herlihy and Nir Shavit won the 2004 Gödel Prize for topological analysis of asynchronous computation. Homology was involved.</p> http://mathoverflow.net/questions/60108/occurrences-of-cohomology-in-other-disciplines-and-or-nature/110622#110622 Answer by Lennart Meier for Occurrences of (co)homology in other disciplines and/or nature Lennart Meier 2012-10-25T06:34:18Z 2012-10-25T06:34:18Z <p>There are some applications of topology/cohomology to combinatorics and combinatoric geometry. One of the earliest examples is surely Lovasz's proof of a bound for the chromatic number of the Kneser graph; he uses the Borsuk-Ulam theorem, which is usually proved by homological methods. A modern exposition can be found <a href="http://arxiv.org/pdf/math/0208072v3.pdf" rel="nofollow">here</a>.</p> <p>Another example is Tveberg's theorem with all its variants on the configuration of points in space (the best results can be found in a recent <a href="http://arxiv.org/pdf/0911.2692.pdf" rel="nofollow">paper of Blagojevic, Matschke and Ziegler</a>. There are many other results in convex geometry/polytope theory which use topological methods and, in particular, cohomology. </p>