Inmathematics,catastrophe theory is a branch ofbifurcation theory in the study ofdynamical systems; it is also a particular special case of more generalsingularity theory ingeometry.

Bifurcation theory studies and classifies phenomena characterized by sudden shifts in behavior arising from small changes in circumstances, analysing how thequalitative 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 andmagnitude of alandslide.

Catastrophe theory originated with the work of the French mathematicianRené Thom in the 1960s, and became very popular due to the efforts ofChristopher 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-definedpotential 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 largerparameter 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.^[1][2] Zahler and Sussmann, in a 1977 article inNature, referred to such applications as being "characterised by incorrect reasoning, far-fetched assumptions, erroneous consequences, and exaggerated claims".^[3] As a result, catastrophe theory has become less popular in applications.^[4]

Catastrophe theory analyzesdegenerate 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 thegerms of the catastrophe geometries. The degeneracy of these critical points can beunfolded by expanding the potential function as aTaylor series in small perturbations of the parameters.

When the degenerate points are not merely accidental, but arestructurally 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 bydiffeomorphism (a smooth transformation whose inverse is also smooth).^{[citation needed]} These seven fundamental types are now presented, with the names that Thom gave them.

Catastrophe theory studies dynamical systems that describe the evolution^[5] of a state variable${\displaystyle x}$  over time${\displaystyle t}$ :

${\displaystyle \dot{x}={\displaystyle \frac{dx}{dt}}=-{\displaystyle \frac{dV(u,x)}{dx}}}$ 

In the above equation,${\displaystyle V}$  is referred to as the potential function, and${\displaystyle u}$  is often a vector or a scalar which parameterise the potential function. The value of${\displaystyle u}$  may change over time, and it can also be referred to as thecontrol variable. In the following examples, parameters like${\displaystyle a,b}$  are such controls.

${\displaystyle V=x^3+ax}$ 

Whena < 0, the potentialV has two extrema - one stable, and one unstable. If the parametera is slowly increased, the system can follow the stable minimum point. But ata = 0 the stable and unstable extrema meet, and annihilate. This is the bifurcation point. Ata > 0 there is no longer a stable solution. If a physical system is followed through a fold bifurcation, one therefore finds that asa reaches 0, the stability of thea < 0 solution is suddenly lost, and the system will make a sudden transition to a new, very different behaviour.This bifurcation value of the parametera is sometimes called the "tipping point".

${\displaystyle V=x^4+a{x}^2+bx}$ 

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 acurve (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 increasingb and then decreasing it, one can therefore observehysteresis 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 spacea < 0. Asa is increased, the hysteresis loops become smaller and smaller, until abovea = 0 they disappear altogether (the cusp catastrophe), and there is only one stable solution.

One can also consider what happens if one holdsb constant and variesa. In the symmetrical caseb = 0, one observes apitchfork bifurcation asa is reduced, with one stable solution suddenly splitting into two stable solutions and one unstable solution as the physical system passes toa < 0 through the cusp point (0,0) (an example ofspontaneous 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.^[6] The suggestion is that at moderate stress (a > 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 (a < 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.^[7]

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

Other applications include theouter sphere electron transfer frequently encountered in chemical and biological systems,^[8] modelling the dynamics ofcloud condensation nuclei in the atmosphere,^[9] and modelling real estate prices.^[10]

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 detectingblack holes and thedark matter of the universe, via the phenomenon ofgravitational lensing producing multiple images of distantquasars.^[11]

The remaining simple catastrophe geometries are very specialised in comparison.

${\displaystyle V=x^5+a{x}^3+b{x}^2+cx}$ 

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 ofx. For values ofa > 0, beyond the swallowtail, there is either one maximum-minimum pair, or none at all, depending on the values ofb andc. Two of the surfaces of fold bifurcations, and the two lines of cusp bifurcations where they meet fora < 0, therefore disappear at the swallowtail point, to be replaced with only a single surface of fold bifurcations remaining.Salvador Dalí's last painting,The Swallow's Tail, was based on this catastrophe.

${\displaystyle V=x^6+a{x}^4+b{x}^3+c{x}^2+dx}$ 

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 whena > 0.

Umbilic catastrophes are examples of corank 2 catastrophes. They can be observed inoptics in thefocal surfaces 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.

${\displaystyle V=x^3+y^3+axy+bx+cy}$ 

${\displaystyle V=\frac{x^3}{3}-x{y}^2+a(x^2+y^2)+bx+cy}$ 

${\displaystyle V=x^{2}y+y^4+a{x}^2+b{y}^2+cx+dy}$ 

Vladimir Arnold gave the catastrophes theADE classification, due to a deep connection withsimple Lie groups.^{[citation needed]}

• A₀ - a non-singular point:${\displaystyle V=x}$ .
• A₁ - a local extremum, either a stable minimum or unstable maximum${\displaystyle V=\pm x^2+ax}$ .
• A₂ - the fold
• A₃ - the cusp
• A₄ - the swallowtail
• A₅ - the butterfly
• A_k - a representative of an infinite sequence of one variable forms${\displaystyle V=x^{k+1}+\cdots }$ 
• D₄⁻ - the elliptical umbilic
• D₄⁺ - the hyperbolic umbilic
• D₅ - the parabolic umbilic
• D_k - a representative of an infinite sequence of further umbilic forms
• E₆ - the symbolic umbilic${\displaystyle V=x^3+y^4+ax{y}^2+bxy+cx+dy+e{y}^2}$ 
• E₇
• E₈

There are objects in singularity theory which correspond to most of the other simple Lie groups.

As predicted by catastrophe theory, singularities are generic, and stable under perturbation. This explains how the bright lines and surfaces are stable under perturbation. Thecaustics 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.^[12]

The edge of therainbow, for example, has a fold catastrophe. Due to the wave nature of light, the catastrophe has fine diffraction details described by theAiry 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.^[13][14] The same Airy function fold catastrophe can be seen innuclear-nuclear scattering ("nuclear rainbow").^[15]

The cusp catastrophe is the next-simplest to observe. Due to the wave nature of light, the catastrophe has fine diffraction details described by thePearcey function.^[16] Higher-order catastrophes, such as the swallowtail and the butterfly, have also been observed.^[17]

• Arnold, Vladimir Igorevich (1992)Catastrophe Theory, 3rd ed. Berlin: Springer-Verlag
• 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: WestviewISBN 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: BirkhäuserISBN 0-8176-3668-4
• Postle, Denis (1980)Catastrophe Theory – Predict and avoid personal disasters, Fontana PaperbacksISBN 0-00-635559-5
• Poston, Tim andStewart, Ian (1998)Catastrophe: Theory and Its Applications, New York: DoverISBN 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
• Thom, René (1989)Structural Stability and Morphogenesis: An Outline of a General Theory of Models, Reading, MA: Addison-WesleyISBN 0-201-09419-3
• Woodcock, Alexander Edward Richard and Davis, Monte. (1978)Catastrophe Theory, New York: E. P. DuttonISBN 0-525-07812-6
• Zeeman, E.C. (1977)Catastrophe Theory-Selected Papers 1972–1977, Reading, MA: Addison-Wesley

• CompLexicon: Catastrophe Theory
• Catastrophe teacher
• Java simulation of Zeeman's catastrophe machine