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{{short description|Area of mathematics}}
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{{Technical|date=September 2018}}

In [[mathematics]], '''catastrophe theory''' is a branch of [[bifurcation theory]] in the study of [[dynamical system]]s; it is also a particular special case of more general [[singularity theory]] in [[geometry]].

Bifurcation theory studies and classifies phenomena characterized by sudden shifts in behavior arising from small changes in circumstances, analysing how the [[qualitative data|qualitative]] nature of equation solutions depends on the parameters that appear in the equation.  This may lead to sudden and dramatic changes, for example the unpredictable timing and [[magnitude (mathematics)|magnitude]] of a [[landslide]].

Catastrophe theory originated with the work of the French mathematician [[René Thom]] in the 1960s, and became very popular due to the efforts of [[Christopher Zeeman]] in the 1970s. It considers the special case where the long-run stable equilibrium can be identified as the minimum of a smooth, well-defined [[scalar potential|potential]] function ([[Lyapunov function]]).
Small changes in certain parameters of a nonlinear system can cause equilibria to appear or disappear, or to change from attracting to repelling and vice versa, leading to large and sudden changes of the behaviour of the system.  However, examined in a larger [[parameter space]], catastrophe theory reveals that such bifurcation points tend to occur as part of well-defined qualitative geometrical structures.

In the late 1970s, applications of catastrophe theory to areas outside its scope began to be criticized, especially in biology and social sciences.<ref>{{cite encyclopedia |last1=Murray |first1=Stacey R. |title=The Rise and Fall of Catastrophe Theory |encyclopedia=[[Encyclopedia.com]] |url=https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/rise-and-fall-catastrophe-theory |access-date=2 November 2021}}</ref><ref>{{cite book |last1=Horgan |first1=John |title=The End of Science: Facing the Limits of Knowledge in the Twilight of the Scientific Age |date=2015 |publisher=Basic Books |location=New York |isbn=978-0-465-05085-7 |page=213}}</ref> Zahler and Sussmann, in a 1977 article in ''[[Nature (journal)|Nature]]'', referred to such applications as being "characterised by incorrect reasoning, far-fetched assumptions, erroneous consequences, and exaggerated claims".<ref>{{cite journal |doi=10.1038/269759a0 |issn=1476-4687 |volume=269 |issue=5631 |pages=759–763 |last1=Zahler |first1=Raphael S. |last2=Sussmann |first2=Hector J. |title=Claims and accomplishments of applied catastrophe theory |journal=Nature |accessdate=2021-11-02 |date=1977 |bibcode=1977Natur.269..759Z |s2cid=4205198 |url=https://www.nature.com/articles/269759a0}}</ref> As a result, catastrophe theory has become less popular in applications.<ref>{{cite journal |last1=Rosser |first1=J. Barkley |title=The rise and fall of catastrophe theory applications in economics: Was the baby thrown out with the bathwater? |journal=Journal of Economic Dynamics and Control |date=October 2007 |volume=31 |issue=10 |pages=3255–3280 |doi=10.1016/j.jedc.2006.09.013}}</ref>

==Elementary catastrophes==
Catastrophe theory analyzes ''degenerate critical points'' of the potential function — points where not just the first derivative, but one or more higher derivatives of the potential function are also zero.  These are called the [[germ (mathematics)|germs]] of the catastrophe geometries. The degeneracy of these critical points can be ''unfolded'' by expanding the potential function as a [[Taylor series]] in small perturbations of the parameters.

When the degenerate points are not merely accidental, but are [[structural stability|structurally stable]], the degenerate points exist as organising centres for particular geometric structures of lower degeneracy, with critical features in the parameter space around them.  If the potential function depends on two or fewer active variables, and four or fewer active parameters, then there are only seven generic structures for these bifurcation geometries, with corresponding standard forms into which the Taylor series around the catastrophe germs can be transformed by [[diffeomorphism]] (a smooth transformation whose inverse is also smooth).{{Citation needed|date=May 2010}}  These seven fundamental types are now presented, with the names that Thom gave them.

==Potential functions of one active variable==
Catastrophe theory studies dynamical systems that describe the evolution<ref>{{cite encyclopedia |last1=Wagenmakers |first1=E. J. |last2=van der Maas |first2=H. L. J. |last3=Molenaar |first3=P. C. M. |date=2005 |title=Fitting the cusp catastrophe model |encyclopedia=Encyclopedia of Statistics in Behavioral Science |url=https://dare.uva.nl/search?identifier=f85f2044-56b3-498f-980c-75939d89a737 }}</ref> of a state variable <math>x</math>  over time <math>t</math>:
:<math>\dot{x} = \dfrac{dx}{dt} = -\dfrac{dV(u,x)}{dx}</math>

In the above equation, <math>V</math> is referred to as the potential function, and <math>u</math> is often a vector or a scalar which parameterise the potential function. The value of <math>u</math> may change over time, and it can also be referred to as the [[control theory|control]] variable. In the following examples, parameters like <math>a,b</math> are such controls.

=== Fold catastrophe ===
[[File:Fold_catastrophe_animation.gif|thumb|Fold catastrophe, with surface <math>z = -y^2- 0.1 x^4</math>.]]
[[File:fold bifurcation.svg|frame|right|Stable and unstable pair of extrema disappear at a fold bifurcation]]
:<math>V = x^3 + ax\,</math>

When {{nowrap|''a'' &lt; 0}}, the potential ''V'' has two extrema - one stable, and one unstable.  If the parameter ''a'' is slowly increased, the system can follow the stable minimum point.  But at {{nowrap|''a'' {{=}} 0}} the stable and unstable extrema meet, and annihilate.  This is the bifurcation point.  At {{nowrap|''a'' &gt; 0}} there is no longer a stable solution.  If a physical system is followed through a fold bifurcation, one therefore finds that as ''a'' reaches 0, the stability of the {{nowrap|''a'' &lt; 0}} solution is suddenly lost, and the system will make a sudden transition to a new, very different behaviour.  {{anchor|Tipping point}}This bifurcation value of the parameter ''a'' is sometimes called the "[[tipping point (physics)|tipping point]]".

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=== Cusp catastrophe===
{{multiple image
| perrow = 1/2
| total_width = 350
| image1 = cusp catastrophe.svg
| caption1 = Diagram of cusp catastrophe, showing curves (brown, red) of ''x'' satisfying ''dV''/''dx'' = ''0'' for parameters (''a'',''b''), drawn for parameter ''b'' continuously varied, for several values of parameter ''a''.<p>
Outside the cusp locus of bifurcations (blue), for each point (''a'',''b'') in parameter space there is only one extremising value of ''x''. Inside the cusp, there are two different values of ''x'' giving local minima of ''V''(''x'') for each (''a'',''b''), separated by a value of ''x'' giving a local maximum.</p>
| image2 = cusp shape.svg
| caption2 = Cusp shape in parameter space (''a'',''b'') near the catastrophe point, showing the locus of fold bifurcations separating the region with two stable solutions from the region with one.
| image3 = pitchfork bifurcation left.svg
| caption3 = Pitchfork bifurcation at {{nowrap|''a'' {{=}} 0}} on the surface {{nowrap|''b'' {{=}} 0}}
}}
[[File:Cusp catastrophe animation gif.gif|thumb|Cusp catastrophe, with surface <math>z = -0.1 (x^4 + y^4) - y^3 + xy</math>.]]<math>V = x^4 + ax^2 + bx \,</math>

The cusp geometry is very common when one explores what happens to a fold bifurcation if a second parameter, ''b'', is added to the control space.  Varying the parameters, one finds that there is now a ''curve'' (blue) of points in (''a'',''b'') space where stability is lost, where the stable solution will suddenly jump to an alternate outcome.

But in a cusp geometry the bifurcation curve loops back on itself, giving a second branch where this alternate solution itself loses stability, and will make a jump back to the original solution set.  By repeatedly increasing ''b'' and then decreasing it, one can therefore observe [[hysteresis]] loops, as the system alternately follows one solution, jumps to the other, follows the other back, and then jumps back to the first.

However, this is only possible in the region of parameter space {{nowrap|''a'' &lt; 0}}.  As ''a'' is increased, the hysteresis loops become smaller and smaller, until above {{nowrap|''a'' {{=}} 0}} they disappear altogether (the cusp catastrophe), and there is only one stable solution.

One can also consider what happens if one holds ''b'' constant and varies ''a''.  In the symmetrical case {{nowrap|''b'' {{=}} 0}}, one observes a [[pitchfork bifurcation]] as ''a'' is reduced, with one stable solution suddenly splitting into two stable solutions and one unstable solution as the physical system passes to {{nowrap|''a'' &lt; 0}} through the cusp point (0,0) (an example of [[spontaneous symmetry breaking]]).  Away from the cusp point, there is no sudden change in a physical solution being followed: when passing through the curve of fold bifurcations, all that happens is an alternate second solution becomes available.

A famous suggestion is that the cusp catastrophe can be used to model the behaviour of a stressed dog, which may respond by becoming cowed or becoming angry.<ref>[[E.C. Zeeman]], [http://www.gaianxaos.com/pdf/dynamics/zeeman-catastrophe_theory.pdf Catastrophe Theory], ''[[Scientific American]]'', April 1976; pp. 65–70, 75–83</ref>  The suggestion is that at moderate stress ({{nowrap|''a'' &gt; 0}}), the dog will exhibit a smooth transition of response from cowed to angry, depending on how it is provoked.  But higher stress levels correspond to moving to the region ({{nowrap|''a'' &lt; 0}}).  Then, if the dog starts cowed, it will remain cowed as it is irritated more and more, until it reaches the 'fold' point, when it will suddenly, discontinuously snap through to angry mode.  Once in 'angry' mode, it will remain angry, even if the direct irritation parameter is considerably reduced.

A simple mechanical system, the "Zeeman Catastrophe Machine", nicely illustrates a cusp catastrophe.  In this device, smooth variations in the position of the end of a spring can cause sudden changes in the rotational position of an attached wheel.<ref>Cross, Daniel J., [https://www.geneva.edu/dept/chemistry-math-physics/physics-research/catastrophe-machine/index Interactive rendering of Zeeman's Catastrophe Machine]</ref>

Catastrophic failure of a [[complex system]] with parallel redundancy can be evaluated based on the relationship between local and external stresses. The model of the [[structural fracture mechanics]] is similar to the cusp catastrophe behavior. The model predicts reserve ability of a complex system.

Other applications include the [[outer sphere electron transfer]] frequently encountered in chemical and biological systems,<ref>{{cite journal |last=Xu |first=F |title=Application of catastrophe theory to the ∆G<sup>≠</sup> to -∆G relationship in electron transfer reactions |journal=Zeitschrift für Physikalische Chemie |series=Neue Folge |volume=166 |pages=79–91 |date=1990 |doi=10.1524/zpch.1990.166.Part_1.079 |s2cid=101078817}}</ref> modelling the dynamics of [[cloud condensation nuclei]] in the atmosphere,<ref>{{cite journal |last=Arabas |first=S | title=On the CCN (de)activation nonlinearities  |doi=10.5194/npg-24-535-2017 |date=2017 |author2=Shima, S. |journal=Nonlinear Processes in Geophysics |volume=24 |issue=3 |pages=535–542|arxiv=1608.08187 |bibcode=2017NPGeo..24..535A |s2cid=24669360 |doi-access=free }}</ref> and modelling real estate prices.<ref>{{cite journal |last=Bełej |first=Mirosław |author2=Kulesza, Sławomir |title=Modeling the Real Estate Prices in Olsztyn under Instability Conditions |journal=Folia Oeconomica Stetinensia |volume=11 |issue=1 |pages=61–72 |doi=10.2478/v10031-012-0008-7 |year=2012 |doi-access=free}}</ref>

Fold bifurcations and the cusp geometry are by far the most important practical consequences of catastrophe theory.  They are patterns which reoccur again and again in physics, engineering and mathematical modelling.
They produce the strong gravitational lensing events and provide astronomers with one of the methods used for detecting [[black holes]] and the [[dark matter]] of the universe, via the phenomenon of [[gravitational lensing]] producing multiple images of distant [[quasar]]s.<ref>A.O. Petters, H. Levine and J. Wambsganss, Singularity Theory and Gravitational Lensing", Birkhäuser Boston (2001)</ref>

The remaining simple catastrophe geometries are very specialised in comparison.

===Swallowtail catastrophe===
[[File:Swallowtail catastrophe animation gif.gif|thumb|Swallowtail catastrophe, with surface <math>z = -y^4 - 0.1x^4 + (1-x^2)y^2 + 0.4xy</math>]]
[[File:Smallow tail.jpg|thumb|right|160px|Swallowtail catastrophe surface]]
:<math>V = x^5 + ax^3 + bx^2 + cx \, </math>

The control parameter space is three-dimensional.  The bifurcation set in parameter space is made up of three surfaces of fold bifurcations, which meet in two lines of cusp bifurcations, which in turn meet at a single swallowtail bifurcation point.

As the parameters go through the surface of fold bifurcations, one minimum and one maximum of the potential function disappear.  At the cusp bifurcations, two minima and one maximum are replaced by one minimum; beyond them the fold bifurcations disappear.  At the swallowtail point, two minima and two maxima all meet at a single value of ''x''.  For values of {{nowrap|''a'' &gt; 0}}, beyond the swallowtail, there is either one maximum-minimum pair, or none at all, depending on the values of ''b'' and ''c''. Two of the surfaces of fold bifurcations, and the two lines of cusp bifurcations where they meet for {{nowrap|''a'' &lt; 0}}, therefore disappear at the swallowtail point, to be replaced with only a single surface of fold bifurcations remaining. [[Salvador Dalí|Salvador Dalí's]] last painting, ''[[The Swallow's Tail]]'', was based on this catastrophe.

===Butterfly catastrophe===
[[File:Butterfly_catastrophe_animation_gif.gif|thumb|Butterfly catastrophe, with surface <math>z=-20x^5 + 4x^3 - 2xy - 0.5 (x^4 + y^4)</math>.]]
:<math>V = x^6 + ax^4 + bx^3 + cx^2 + dx \, </math>

Depending on the parameter values, the potential function may have three, two, or one different local minima, separated by the loci of fold bifurcations.  At the butterfly point, the different 3-surfaces of fold bifurcations, the 2-surfaces of cusp bifurcations, and the lines of swallowtail bifurcations all meet up and disappear, leaving a single cusp structure remaining when {{nowrap|''a'' &gt; 0}}.

==Potential functions of two active variables==
[[File:Hyperbolic umbilic focal surface.png|thumb|A surface with a hyperbolic umbilic and its focal surface. The hyperbolic umbilic catastrophe is just the upper part of this image.]]
[[File:Elliptical umbilic focal surface.png|thumb|right|A surface with an elliptical umbilic, and its focal surface. The elliptic umbilic catastrophe is just the upper part of this image.]]

Umbilic catastrophes are examples of corank 2 catastrophes. They can be observed in [[optics]] in the [[focal surface]]s created by light reflecting off a surface in three dimensions and are intimately connected with the geometry of nearly spherical surfaces: [[umbilical point]].
Thom proposed that the hyperbolic umbilic catastrophe modeled the breaking of a wave and the elliptical umbilic modeled the creation of hair-like structures.

===Hyperbolic umbilic catastrophe===
:<math>V = x^3 + y^3 + axy + bx +cy \, </math>

===Elliptic umbilic catastrophe===
:<math>V = \frac{x^3}{3} - xy^2 + a(x^2+y^2) + bx + cy \, </math>

===Parabolic umbilic catastrophe===
:<math>V = x^2y + y^4 + ax^2 + by^2 + cx + dy \, </math>

==Arnold's notation==
[[Vladimir Arnold]] gave the catastrophes the [[ADE classification]], due to a deep connection with [[simple Lie group]]s. {{Citation needed|date=June 2020}}
*''A''<sub>0</sub> - a non-singular point: <math>V = x</math>.
*''A''<sub>1</sub> - a local extremum, either a stable minimum or unstable maximum <math>V = \pm x^2 + a x</math>.
*''A''<sub>2</sub> - the fold
*''A''<sub>3</sub> - the cusp
*''A''<sub>4</sub> - the swallowtail
*''A''<sub>5</sub> - the butterfly
*''A''<sub>k</sub> - a representative of an infinite sequence of one variable forms <math>V=x^{k+1}+\cdots</math>
*''D''<sub>4</sub><sup>−</sup> - the elliptical umbilic
*''D''<sub>4</sub><sup>+</sup> - the hyperbolic umbilic
*''D''<sub>5</sub> - the parabolic umbilic
*''D''<sub>k</sub> - a representative of an infinite sequence of further umbilic forms
*''E''<sub>6</sub> - the symbolic umbilic <math>V = x^3+y^4+a x y^2 +bxy+cx+dy+ey^2</math>
*''E''<sub>7</sub>
*''E''<sub>8</sub>
There are objects in singularity theory which correspond to most of the other simple Lie groups.

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== Optics ==
[[File:Caustic Shallow Water.webm|thumb|Caustics underwater are catastrophes, and their singular points are the same even as the surface of the water is everchanging.]]
[[File:Supernumerary-rainbows-jb.jpg|thumb|The edge of the rainbow is a fold catastrophe, and so it has a diffraction pattern described by the Airy function. This is generic and stable for any shape of the water droplet.]]

As predicted by catastrophe theory, singularities are generic, and stable under perturbation. This explains how the bright lines and surfaces are stable under perturbation. The [[Caustic (optics)|caustics]] one sees at the bottom of a swimming pool, for example, have a distinctive texture and only has a few types of singular points, even though the surface of the water is ever changing.<ref>{{Citation |last1=Berry |first1=M. V. |title=IV Catastrophe Optics: Morphologies of Caustics and Their Diffraction Patterns |date=1980-01-01 |volume=18 |pages=257–346 |editor-last=Wolf |editor-first=E. |url=https://www.sciencedirect.com/science/article/pii/S0079663808702154 |access-date=2024-04-25 |publisher=Elsevier |last2=Upstill |first2=C.|journal=Progress in Optics |doi=10.1016/S0079-6638(08)70215-4 |bibcode=1980PrOpt..18..257B |isbn=978-0-444-85445-2 }}</ref>

The edge of the [[rainbow]], for example, has a fold catastrophe. Due to the wave nature of light, the catastrophe has fine diffraction details described by the [[Airy function]]. This is a generic result and does not depend on the precise shape of the water droplet, and so the edge of the rainbow always has the shape of an Airy function.<ref>{{Cite journal |last=Adam |first=John A. |date=2002-01-01 |title=The mathematical physics of rainbows and glories |url=https://www.sciencedirect.com/science/article/pii/S037015730100076X |journal=Physics Reports |volume=356 |issue=4 |pages=229–365 |doi=10.1016/S0370-1573(01)00076-X |bibcode=2002PhR...356..229A |issn=0370-1573}}</ref><ref>{{Cite web |title=AMS :: Feature Column :: The Mathematics of the Rainbow, Part II |url=https://www.ams.org/publicoutreach/feature-column/fc-2015-02 |access-date=2024-04-25 |website=American Mathematical Society }}</ref> The same Airy function fold catastrophe can be seen in [[nuclear-nuclear scattering]] ("nuclear rainbow").<ref>{{Cite journal |last1=Khoa |first1=Dao T |last2=Oertzen |first2=W von |last3=Bohlen |first3=H G |last4=Ohkubo |first4=S |date=2007-03-01 |title=Nuclear rainbow scattering and nucleus–nucleus potential |url=https://iopscience.iop.org/article/10.1088/0954-3899/34/3/R01 |journal=Journal of Physics G: Nuclear and Particle Physics |volume=34 |issue=3 |pages=R111–R164 |doi=10.1088/0954-3899/34/3/R01 |issn=0954-3899|arxiv=nucl-th/0612100 }}</ref>

The cusp catastrophe is the next-simplest to observe. Due to the wave nature of light, the catastrophe has fine diffraction details described by the [[Pearcey integral|Pearcey function]].<ref>{{Cite thesis |title=The Pearcey function and the cusp catastrophe |url=https://macsphere.mcmaster.ca/handle/11375/20436 |date=November 2016 |degree=Thesis |first=Darryl |last=MacBeath}}</ref> Higher-order catastrophes, such as the swallowtail and the butterfly, have also been observed.<ref>{{Cite journal |last1=Zannotti |first1=Alessandro |last2=Diebel |first2=Falko |last3=Boguslawski |first3=Martin |last4=Denz |first4=Cornelia |date=May 2017 |title=Optical catastrophes of the swallowtail and butterfly beams |url=https://dx.doi.org/10.1088/1367-2630/aa6ecd |journal=New Journal of Physics |volume=19 |issue=5 |pages=053004 |doi=10.1088/1367-2630/aa6ecd |issn=1367-2630|arxiv=1703.07716 |bibcode=2017NJPh...19e3004Z }}</ref>
[[File:Circle_caustic.png|thumb|Cusp catastrophe caustic, generated from a circle and parallel rays.]]
[[File:A_photograph_of_a_cusp_caustic.png|thumb|A photograph of a cusp caustic produced by illuminating a flat surface with a laser beam through a droplet of water. This is a Pearcey function and stable under perturbations.]]

==See also==
{{Div col|colwidth=20em}}
* [[Broken symmetry]]
* [[Butterfly effect]]
* [[Chaos theory]]
* [[Domino effect]]
* [[Inflection point]]
* [[Morphology (biology)|Morphology]]
* [[Phase transition]]
* [[Punctuated equilibrium]]
* [[Snowball effect]]
* [[Spontaneous symmetry breaking]]
{{Div col end}}

==References==
{{Reflist|25em}}

==Bibliography==
* [[Vladimir Arnold|Arnold, Vladimir Igorevich]] (1992) ''Catastrophe Theory'', 3rd ed. Berlin: Springer-Verlag
* [[Valentin Afraimovich|V. S. Afrajmovich]], V. I. Arnold, et al., ''Bifurcation Theory And Catastrophe Theory'', {{ISBN|3-540-65379-1}}
* Bełej, M. Kulesza, S. (2013) M"odeling the Real Estate Prices in Olsztyn under Instability Conditions", ''Folia Oeconomica Stetinensia'' 11(1): 61–72, ISSN (Online) 1898–0198, ISSN (Print) 1730–4237, {{doi|10.2478/v10031-012-0008-7}}
* Castrigiano, Domenico P. L. and Hayes, Sandra A. (2004) ''Catastrophe Theory'', second edition, Boulder: Westview {{ISBN|0-8133-4126-4}}
* Gilmore, Robert (1993) ''Catastrophe Theory for Scientists and Engineers'', New York: Dover
* Petters, Arlie O., Levine, Harold and Wambsganss, Joachim (2001) ''Singularity Theory and Gravitational Lensing'', Boston: Birkhäuser {{ISBN|0-8176-3668-4}}
* Postle, Denis (1980) ''Catastrophe Theory – Predict and avoid personal disasters'', Fontana Paperbacks {{ISBN|0-00-635559-5}}
* [[Tim Poston|Poston, Tim]] and [[Ian Stewart (mathematician)|Stewart, Ian]] (1998) ''Catastrophe: Theory and Its Applications'', New York: Dover {{ISBN|0-486-69271-X}}
* Sanns, Werner (2000) ''Catastrophe Theory with Mathematica: A Geometric Approach'', Germany: DAV
* Saunders, Peter Timothy (1980) ''An Introduction to Catastrophe Theory'', Cambridge, England: Cambridge University Press
* [[René Thom|Thom, René]] (1989) ''Structural Stability and Morphogenesis: An Outline of a General Theory of Models'', Reading, MA: Addison-Wesley {{ISBN|0-201-09419-3}}
* Woodcock, Alexander Edward Richard and Davis, Monte.  (1978) ''Catastrophe Theory'', New York: E. P. Dutton {{ISBN|0-525-07812-6}}
* [[Erik Christopher Zeeman|Zeeman, E.C.]] (1977) ''Catastrophe Theory-Selected Papers 1972&ndash;1977'', Reading, MA: Addison-Wesley

==External links==
*[https://www.exploratorium.edu/complexity/CompLexicon/catastrophe.html CompLexicon: Catastrophe Theory]
*[https://web.archive.org/web/20051128035658/http://perso.wanadoo.fr/l.d.v.dujardin/ct/eng_index.html Catastrophe teacher]
*[https://www.compadre.org/osp/items/detail.cfm?ID=11996 Java simulation of Zeeman's catastrophe machine]

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[[Category:Bifurcation theory]]
[[Category:Singularity theory]]
[[Category:Systems theory]]
[[Category:Chaos theory]]