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{{about|the study of dynamical systems||catastrophe (disambiguation)}}
In [[mathematics]], the '''category of topological spaces''', often denoted '''Top''', is the [[category (category theory)|category]] whose [[object (category theory)|object]]s are [[topological space]]s and whose [[morphism]]s are [[continuous map]]s or some other variant; for example, objects are often assumed to be [[compactly generated space|compactly generated]]. This is a category because the [[function composition|composition]] of two continuous maps is again continuous. The study of '''Top''' and of properties of [[topological space]]s using the techniques of [[category theory]] is known as '''categorical topology'''.


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]].
N.B. Some authors use the name '''Top''' for the category with [[topological manifold]]s as objects and continuous maps as morphisms.


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]].
==As a concrete category==


Catastrophe theory, which 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, considers the special case where the long-run stable equilibrium can be identified with the minimum of a smooth, well-defined [[scalar potential|potential]] function ([[Lyapunov function]]).
Like many categories, the category '''Top''' is a [[concrete category]] (also known as a ''construct''), meaning its objects are [[Set (mathematics)|sets]] with additional structure (i.e. topologies) and its morphisms are [[function (mathematics)|function]]s preserving this structure. There is a natural [[forgetful functor]]
:''U'' : '''Top''' → '''Set'''
to the [[category of sets]] which assigns to each topological space the underlying set and to each continuous map the underlying [[function (mathematics)|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.
The forgetful functor ''U'' has both a [[left adjoint]]
:''D'' : '''Set''' → '''Top'''
which equips a given set with the [[discrete topology]] and a [[right adjoint]]
:''I'' : '''Set''' → '''Top'''
which equips a given set with the [[indiscrete topology]]. Both of these functors are, in fact, [[right inverse]]s to ''U'' (meaning that ''UD'' and ''UI'' are equal to the [[identity functor]] on '''Set'''). Moreover, since any function between discrete or indiscrete spaces is continuous, both of these functors give [[full embedding]]s of '''Set''' into '''Top'''.


== Elementary catastrophes ==
The construct '''Top''' is also ''fiber-complete'' meaning that the [[lattice of topologies|category of all topologies]] on a given set ''X'' (called the ''[[fiber (mathematics)|fiber]]'' of ''U'' above ''X'') forms a [[complete lattice]] when ordered by [[set inclusion|inclusion]]. The [[greatest element]] in this fiber is the discrete topology on ''X'' while the [[least element]] is the indiscrete topology.
Catastrophe theory analyses ''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 (resp. five) or fewer active parameters, then there are only seven (resp. eleven) 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|time=2009-11-3|date=May 2010}}  These seven fundamental types are now presented, with the names that Thom gave them.
The construct '''Top''' is the model of what is called a [[topological category]]. These categories are characterized by the fact that every [[structured source]] <math>(X \to UA_i)_I</math> has a unique [[initial lift]] <math>( A \to A_i)_I</math>. In '''Top''' the initial lift is obtained by placing the [[initial topology]] on the source. Topological categories have many properties in common with '''Top''' (such as fiber-completeness, discrete and indiscrete functors, and unique lifting of limits).


== Potential functions of one active variable ==
==Limits and colimits==
=== Fold catastrophe ===
[[File:fold bifurcation.svg|frame|right|160px|Stable and unstable pair of extrema disappear at a fold bifurcation]]
:<math>V = x^3 + ax\,</math>


At negative values of ''a'', the potential 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".
The category '''Top''' is both [[complete category|complete and cocomplete]], which means that all small [[limit (category theory)|limits and colimit]]s exist in '''Top'''. In fact, the forgetful functor ''U'' : '''Top''' → '''Set''' uniquely lifts both limits and colimits and preserves them as well. Therefore, (co)limits in '''Top''' are given by placing topologies on the corresponding (co)limits in '''Set'''.


<br style="clear:both" />
Specifically, if ''F'' is a [[diagram (category theory)|diagram]] in '''Top''' and  (''L'', φ) is a limit of ''UF'' in '''Set''', the corresponding limit of ''F'' in '''Top''' is obtained by placing the [[initial topology]] on (''L'', φ). Dually, colimits in '''Top''' are obtained by placing the [[final topology]] on the corresponding colimits in '''Set'''.


=== Cusp catastrophe ===
Unlike many algebraic categories, the forgetful functor ''U'' : '''Top''' → '''Set''' does not create or reflect limits since there will typically be non-universal [[cone (category theory)|cones]] in '''Top''' covering universal cones in '''Set'''.
:<math>V = x^4 + ax^2 + bx \,</math>


{| style="float:right; clear:right"
Examples of limits and colimits in '''Top''' include:
| colspan=2 |
[[File:cusp catastrophe.svg|thumb|upright=1.5|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''.


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.]]
*The [[empty set]] (considered as a topological space) is the [[initial object]] of '''Top'''; any [[singleton (mathematics)|singleton]] topological space is a [[terminal object]]. There are thus no [[zero object]]s in '''Top'''.
|- valign="top"
*The [[product (category theory)|product]] in '''Top''' is given by the [[product topology]] on the [[Cartesian product]]. The [[coproduct (category theory)|coproduct]] is given by the [[disjoint union (topology)|disjoint union]] of topological spaces.
| [[File:cusp shape.svg|thumb|upright=0.7|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.]]
*The [[equaliser (mathematics)#In category theory|equalizer]] of a pair of morphisms is given by placing the [[subspace topology]] on the set-theoretic equalizer. Dually, the [[coequalizer]] is given by placing the [[quotient topology]] on the set-theoretic coequalizer.
| [[File:pitchfork bifurcation left.svg|thumb|upright=0.7|Pitchfork bifurcation at {{nowrap|''a'' {{=}} 0}} on the surface {{nowrap|''b'' {{=}} 0}}]]
*[[Direct limit]]s and [[inverse limit]]s are the set-theoretic limits with the [[final topology]] and [[initial topology]] respectively.
|}
*[[Adjunction space]]s are an example of [[pushout (category theory)|pushouts]] in '''Top'''.


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.
==Other properties==
*The [[monomorphism]]s in '''Top''' are the [[injective]] continuous maps, the [[epimorphism]]s are the [[surjective]] continuous maps, and the [[isomorphism]]s are the [[homeomorphism]]s.
*The extremal monomorphisms are (up to isomorphism) the [[subspace topology|subspace]] embeddings. Every extremal monomorphism is [[regular morphism (topology)|regular]].
*The extremal epimorphisms are (essentially) the [[quotient map]]s. Every extremal epimorphism is regular.
*The split monomorphisms are (essentially) the inclusions of [[retract]]s into their ambient space.
*The split epimorphisms are (up to isomorphism) the continuous surjective maps of a space onto one of its retracts.
*There are no [[zero morphism]]s in '''Top''', and in particular the category is not [[preadditive category|preadditive]].
*'''Top''' is not [[cartesian closed category|cartesian closed]] (and therefore also not a [[topos]]) since it does not have [[exponential object]]s for all spaces.


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, then jumps back to the first.
==Relationships to other categories==


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.
*The category of [[pointed topological space]]s '''Top'''<sub>•</sub> is a [[coslice category]] over '''Top'''.
* The [[homotopy category of topological spaces|homotopy category]] '''hTop''' has topological spaces for objects and [[homotopy equivalent|homotopy equivalence classes]] of continuous maps for morphisms. This is a [[quotient category]] of '''Top'''. One can likewise form the pointed homotopy category '''hTop'''<sub>•</sub>.
*'''Top''' contains the important category '''Haus''' of topological spaces with the [[Hausdorff space|Hausdorff]] property as a [[full subcategory]].  The added structure of this subcategory allows for more epimorphisms:  in fact, the epimorphisms in this subcategory are precisely those morphisms with [[dense set|dense]] [[image (mathematics)|images]] in their [[codomain]]s, so that epimorphisms need not be [[surjective]].


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.
== References ==
 
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.
 
Catastrophic failure of a complex system with parallel redundancy can be evaluated based on 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 Neue Folge | volume=166 | pages=79-91 | date=1990}}</ref> and modelling Real Estate Prices.<ref>{{cite journal|last=Bełej|first=Mirosław|coauthors=Kulesza, Sławomir|title=Modeling the Real Estate Prices in Olsztyn under Instability Conditions|journal=Folia Oeconomica Stetinensia|date=1 January 2012|volume=11|issue=1|doi=10.2478/v10031-012-0008-7}}</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 [[quasars]].
<ref>A.O. Petters, H. Levine and J. Wambsganss, Singularity Theory and Gravitational Lensing", Birkauser Boston (2001)</ref>
 
The remaining simple catastrophe geometries are very specialised in comparison, and presented here only for curiosity value.
 
=== Swallowtail catastrophe ===
[[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 ''a>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 ''a<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 ===
:<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 ''a>0''
 
== Potential functions of two active variables ==
Umbilic catastrophes are examples of corank 2 catastrophes. They can be observed in [[optics]] in the focal surfaces created by light reflecting off a surface in three dimensions and are intimately connected with the geometry of nearly spherical surfaces.
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.
*''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.


== See also ==
* Herrlich, Horst: ''Topologische Reflexionen und Coreflexionen''. Springer Lecture Notes in Mathematics 78 (1968).
* [[Broken symmetry]]
* [[Phase transition]]
* [[Domino effect]]
* [[Snowball effect]]
* [[Butterfly effect]]
* [[Spontaneous symmetry breaking]]
* [[Chaos theory]]


== Bibliography ==
* Herrlich, Horst: ''Categorical topology 1971 - 1981''. In: General Topology and its Relations to Modern Analysis and Algebra 5, Heldermann Verlag 1983, pp.&nbsp;279 – 383.
*[[Vladimir Arnold|Arnold, Vladimir Igorevich]]. Catastrophe Theory, 3rd ed. Berlin: Springer-Verlag, 1992.
*[[Valentin Afraimovich|V. S. Afrajmovich]], V. I. Arnold, et al., Bifurcation Theory And Catastrophe Theory, ISBN 3-540-65379-1
*Bełej,M. Kulesza, S. Modeling the Real Estate Prices in Olsztyn under Instability Conditions. Folia Oeconomica Stetinensia. Volume 11, Issue 1, Pages 61–72, ISSN (Online) 1898-0198, ISSN (Print) 1730-4237, DOI: 10.2478/v10031-012-0008-7, 2013
*Castrigiano, Domenico P. L. and Hayes, Sandra A. Catastrophe Theory, 2nd ed. Boulder: Westview, 2004. ISBN 0-8133-4126-4
*Gilmore, Robert. Catastrophe Theory for Scientists and Engineers. New York: Dover, 1993.
*Petters, Arlie O., Levine, Harold and Wambsganss, Joachim. Singularity Theory and Gravitational Lensing. Boston: Birkhauser, 2001. ISBN 0-8176-3668-4
*Postle, Denis. Catastrophe Theory – Predict and avoid personal disasters. Fontana Paperbacks, 1980. ISBN 0-00-635559-5
*[[Tim Poston|Poston, Tim]] and [[Ian Stewart (mathematician)|Stewart, Ian]]. Catastrophe: Theory and Its Applications. New York: Dover, 1998. ISBN 0-486-69271-X.
*Sanns, Werner. Catastrophe Theory with Mathematica: A Geometric Approach. Germany: DAV, 2000.
*Saunders, Peter Timothy. An Introduction to Catastrophe Theory. Cambridge, England: Cambridge University Press, 1980.
*[[René Thom|Thom, René]]. Structural Stability and Morphogenesis: An Outline of a General Theory of Models. Reading, MA: Addison-Wesley, 1989. ISBN 0-201-09419-3.
*Thompson, J. Michael T. Instabilities and Catastrophes in Science and Engineering. New York: Wiley, 1982.
*Woodcock, Alexander Edward Richard and Davis, Monte. Catastrophe Theory. New York: E. P. Dutton, 1978. ISBN 0-525-07812-6.
*[[Erik Christopher Zeeman|Zeeman, E.C.]] Catastrophe Theory-Selected Papers 1972&ndash;1977. Reading, MA: Addison-Wesley, 1977.


== References ==
* Herrlich, Horst & Strecker, George E.: Categorical Topology - its origins, as examplified by the unfolding of the theory of topological reflections and coreflections before 1971. In: Handbook of the History of General Topology (eds. C.E.Aull & R. Lowen), Kluwer Acad. Publ. vol 1 (1997) pp.&nbsp;255 – 341.
{{reflist}}


== External links ==
* Adámek, Jiří, Herrlich, Horst, & Strecker, George E.; (1990). [http://katmat.math.uni-bremen.de/acc/acc.pdf ''Abstract and Concrete Categories''] (4.2MB PDF). Originally publ. John Wiley & Sons. ISBN 0-471-60922-6. (now free on-line edition).
* [http://www.exploratorium.edu/complexity/CompLexicon/catastrophe.html CompLexicon: Catastrophe Theory]
* [http://perso.wanadoo.fr/l.d.v.dujardin/ct/eng_index.html Catastrophe teacher]


[[Category:Bifurcation theory]]
[[Category:Category-theoretic categories|Topological spaces]]
[[Category:Singularity theory]]
[[Category:General topology]]
[[Category:Systems theory]]
[[Category:Chaos theory]]

Revision as of 00:42, 12 August 2014

In mathematics, the category of topological spaces, often denoted Top, is the category whose objects are topological spaces and whose morphisms are continuous maps or some other variant; for example, objects are often assumed to be compactly generated. This is a category because the composition of two continuous maps is again continuous. The study of Top and of properties of topological spaces using the techniques of category theory is known as categorical topology.

N.B. Some authors use the name Top for the category with topological manifolds as objects and continuous maps as morphisms.

As a concrete category

Like many categories, the category Top is a concrete category (also known as a construct), meaning its objects are sets with additional structure (i.e. topologies) and its morphisms are functions preserving this structure. There is a natural forgetful functor

U : Top → Set

to the category of sets which assigns to each topological space the underlying set and to each continuous map the underlying function.

The forgetful functor U has both a left adjoint

D : Set → Top

which equips a given set with the discrete topology and a right adjoint

I : Set → Top

which equips a given set with the indiscrete topology. Both of these functors are, in fact, right inverses to U (meaning that UD and UI are equal to the identity functor on Set). Moreover, since any function between discrete or indiscrete spaces is continuous, both of these functors give full embeddings of Set into Top.

The construct Top is also fiber-complete meaning that the category of all topologies on a given set X (called the fiber of U above X) forms a complete lattice when ordered by inclusion. The greatest element in this fiber is the discrete topology on X while the least element is the indiscrete topology.

The construct Top is the model of what is called a topological category. These categories are characterized by the fact that every structured source has a unique initial lift . In Top the initial lift is obtained by placing the initial topology on the source. Topological categories have many properties in common with Top (such as fiber-completeness, discrete and indiscrete functors, and unique lifting of limits).

Limits and colimits

The category Top is both complete and cocomplete, which means that all small limits and colimits exist in Top. In fact, the forgetful functor U : Top → Set uniquely lifts both limits and colimits and preserves them as well. Therefore, (co)limits in Top are given by placing topologies on the corresponding (co)limits in Set.

Specifically, if F is a diagram in Top and (L, φ) is a limit of UF in Set, the corresponding limit of F in Top is obtained by placing the initial topology on (L, φ). Dually, colimits in Top are obtained by placing the final topology on the corresponding colimits in Set.

Unlike many algebraic categories, the forgetful functor U : Top → Set does not create or reflect limits since there will typically be non-universal cones in Top covering universal cones in Set.

Examples of limits and colimits in Top include:

Other properties

Relationships to other categories

References

  • Herrlich, Horst: Topologische Reflexionen und Coreflexionen. Springer Lecture Notes in Mathematics 78 (1968).
  • Herrlich, Horst: Categorical topology 1971 - 1981. In: General Topology and its Relations to Modern Analysis and Algebra 5, Heldermann Verlag 1983, pp. 279 – 383.
  • Herrlich, Horst & Strecker, George E.: Categorical Topology - its origins, as examplified by the unfolding of the theory of topological reflections and coreflections before 1971. In: Handbook of the History of General Topology (eds. C.E.Aull & R. Lowen), Kluwer Acad. Publ. vol 1 (1997) pp. 255 – 341.
  • Adámek, Jiří, Herrlich, Horst, & Strecker, George E.; (1990). Abstract and Concrete Categories (4.2MB PDF). Originally publ. John Wiley & Sons. ISBN 0-471-60922-6. (now free on-line edition).
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