**Topology** (from the Greek words τόπος , 'place, location', and λόγος, 'study') is the branch of mathematics concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing holes, opening holes, tearing, gluing, or passing through itself.

- Motivation
- History
- Concepts
- Topologies on sets
- Continuous functions and homeomorphisms
- Manifolds
- Topics
- General topology
- Algebraic topology
- Differential topology
- Geometric topology
- Generalizations
- Applications
- Biology
- Computer science
- Physics
- Robotics
- Games and puzzles
- Fiber art
- Resources and research
- Major journals
- Major books
- See also
- References
- Citations
- Bibliography
- Further reading
- External links

A topological space is a set endowed with a structure, called a * topology *, which allows defining continuous deformation of subspaces, and, more generally, all kinds of continuity. Euclidean spaces, and, more generally, metric spaces are examples of topological spaces, as any distance or metric defines a topology. The deformations that are considered in topology are homeomorphisms and homotopies. A property that is invariant under such deformations is a topological property. The following are basic examples of topological properties: the dimension, which allows distinguishing between a line and a surface; compactness, which allows distinguishing between a line and a circle; connectedness, which allows distinguishing a circle from two non-intersecting circles.

The ideas underlying topology go back to Gottfried Wilhelm Leibniz, who in the 17th century envisioned the *geometria situs* and *analysis situs*. Leonhard Euler's Seven Bridges of Königsberg problem and polyhedron formula are arguably the field's first theorems. The term *topology* was introduced by Johann Benedict Listing in the 19th century, although, it was not until the first decades of the 20th century that the idea of a topological space was developed.

The motivating insight behind topology is that some geometric problems depend not on the exact shape of the objects involved, but rather on the way they are put together. For example, the square and the circle have many properties in common: they are both one dimensional objects (from a topological point of view) and both separate the plane into two parts, the part inside and the part outside.

In one of the first papers in topology, Leonhard Euler demonstrated that it was impossible to find a route through the town of Königsberg (now Kaliningrad) that would cross each of its seven bridges exactly once. This result did not depend on the lengths of the bridges or on their distance from one another, but only on connectivity properties: which bridges connect to which islands or riverbanks. This Seven Bridges of Königsberg problem led to the branch of mathematics known as graph theory.

Similarly, the hairy ball theorem of algebraic topology says that "one cannot comb the hair flat on a hairy ball without creating a cowlick." This fact is immediately convincing to most people, even though they might not recognize the more formal statement of the theorem, that there is no nonvanishing continuous tangent vector field on the sphere. As with the *Bridges of Königsberg*, the result does not depend on the shape of the sphere; it applies to any kind of smooth blob, as long as it has no holes.

To deal with these problems that do not rely on the exact shape of the objects, one must be clear about just what properties these problems do rely on. From this need arises the notion of homeomorphism. The impossibility of crossing each bridge just once applies to any arrangement of bridges homeomorphic to those in Königsberg, and the hairy ball theorem applies to any space homeomorphic to a sphere.

Intuitively, two spaces are homeomorphic if one can be deformed into the other without cutting or gluing. A traditional joke is that a topologist cannot distinguish a coffee mug from a doughnut, since a sufficiently pliable doughnut could be reshaped to a coffee cup by creating a dimple and progressively enlarging it, while shrinking the hole into a handle.^{ [1] }

Homeomorphism can be considered the most basic topological equivalence. Another is homotopy equivalence. This is harder to describe without getting technical, but the essential notion is that two objects are homotopy equivalent if they both result from "squishing" some larger object.

Topology, as a well-defined mathematical discipline, originates in the early part of the twentieth century, but some isolated results can be traced back several centuries.^{ [2] } Among these are certain questions in geometry investigated by Leonhard Euler. His 1736 paper on the Seven Bridges of Königsberg is regarded as one of the first practical applications of topology.^{ [2] } On 14 November 1750, Euler wrote to a friend that he had realized the importance of the *edges* of a polyhedron. This led to his polyhedron formula, *V* − *E* + *F* = 2 (where V, E, and F respectively indicate the number of vertices, edges, and faces of the polyhedron). Some authorities regard this analysis as the first theorem, signaling the birth of topology.^{ [3] }

Further contributions were made by Augustin-Louis Cauchy, Ludwig Schläfli, Johann Benedict Listing, Bernhard Riemann and Enrico Betti.^{ [4] } Listing introduced the term "Topologie" in *Vorstudien zur Topologie*, written in his native German, in 1847, having used the word for ten years in correspondence before its first appearance in print.^{ [5] } The English form "topology" was used in 1883 in Listing's obituary in the journal *Nature* to distinguish "qualitative geometry from the ordinary geometry in which quantitative relations chiefly are treated".^{ [6] }

Their work was corrected, consolidated and greatly extended by Henri Poincaré. In 1895, he published his ground-breaking paper on * Analysis Situs *, which introduced the concepts now known as homotopy and homology, which are now considered part of algebraic topology.^{ [4] }

Manifold | Euler num | Orientability | Betti numbers | Torsion coefficient (1-dim) | ||
---|---|---|---|---|---|---|

b_{0} | b_{1} | b_{2} | ||||

Sphere | 2 | Orientable | 1 | 0 | 1 | none |

Torus | 0 | Orientable | 1 | 2 | 1 | none |

2-holed torus | −2 | Orientable | 1 | 4 | 1 | none |

g-holed torus (genus g) | 2 − 2g | Orientable | 1 | 2g | 1 | none |

Projective plane | 1 | Non-orientable | 1 | 0 | 0 | 2 |

Klein bottle | 0 | Non-orientable | 1 | 1 | 0 | 2 |

Sphere with c cross-caps (c > 0) | 2 − c | Non-orientable | 1 | c − 1 | 0 | 2 |

2-Manifold with g holes and c cross-caps ( c > 0) | 2 − (2g + c) | Non-orientable | 1 | (2g + c) − 1 | 0 | 2 |

Unifying the work on function spaces of Georg Cantor, Vito Volterra, Cesare Arzelà, Jacques Hadamard, Giulio Ascoli and others, Maurice Fréchet introduced the metric space in 1906.^{ [7] } A metric space is now considered a special case of a general topological space, with any given topological space potentially giving rise to many distinct metric spaces. In 1914, Felix Hausdorff coined the term "topological space" and gave the definition for what is now called a Hausdorff space.^{ [8] } Currently, a topological space is a slight generalization of Hausdorff spaces, given in 1922 by Kazimierz Kuratowski.^{ [9] }

Modern topology depends strongly on the ideas of set theory, developed by Georg Cantor in the later part of the 19th century. In addition to establishing the basic ideas of set theory, Cantor considered point sets in Euclidean space as part of his study of Fourier series. For further developments, see point-set topology and algebraic topology.

The 2022 Abel Prize was awarded to Dennis Sullivan "for his groundbreaking contributions to topology in its broadest sense, and in particular its algebraic, geometric and dynamical aspects".^{ [10] }

The term *topology* also refers to a specific mathematical idea central to the area of mathematics called topology. Informally, a topology describes how elements of a set relate spatially to each other. The same set can have different topologies. For instance, the real line, the complex plane, and the Cantor set can be thought of as the same set with different topologies.

Formally, let X be a set and let τ be a family of subsets of X. Then τ is called a topology on X if:

- Both the empty set and X are elements of τ.
- Any union of elements of τ is an element of τ.
- Any intersection of finitely many elements of τ is an element of τ.

If τ is a topology on X, then the pair (*X*, *τ*) is called a topological space. The notation *X*_{τ} may be used to denote a set X endowed with the particular topology τ. By definition, every topology is a π-system.

The members of τ are called *open sets* in X. A subset of X is said to be closed if its complement is in τ (that is, its complement is open). A subset of X may be open, closed, both (a clopen set), or neither. The empty set and X itself are always both closed and open. An open subset of X which contains a point x is called an open neighborhood of x.

A function or map from one topological space to another is called *continuous* if the inverse image of any open set is open. If the function maps the real numbers to the real numbers (both spaces with the standard topology), then this definition of continuous is equivalent to the definition of continuous in calculus. If a continuous function is one-to-one and onto, and if the inverse of the function is also continuous, then the function is called a homeomorphism and the domain of the function is said to be homeomorphic to the range. Another way of saying this is that the function has a natural extension to the topology. If two spaces are homeomorphic, they have identical topological properties, and are considered topologically the same. The cube and the sphere are homeomorphic, as are the coffee cup and the doughnut. However, the sphere is not homeomorphic to the doughnut.

While topological spaces can be extremely varied and exotic, many areas of topology focus on the more familiar class of spaces known as manifolds. A *manifold* is a topological space that resembles Euclidean space near each point. More precisely, each point of an n-dimensional manifold has a neighborhood that is homeomorphic to the Euclidean space of dimension n. Lines and circles, but not figure eights, are one-dimensional manifolds. Two-dimensional manifolds are also called surfaces, although not all surfaces are manifolds. Examples include the plane, the sphere, and the torus, which can all be realized without self-intersection in three dimensions, and the Klein bottle and real projective plane, which cannot (that is, all their realizations are surfaces that are not manifolds).

General topology is the branch of topology dealing with the basic set-theoretic definitions and constructions used in topology.^{ [11] }^{ [12] } It is the foundation of most other branches of topology, including differential topology, geometric topology, and algebraic topology. Another name for general topology is point-set topology.

The basic object of study is topological spaces, which are sets equipped with a topology, that is, a family of subsets, called *open sets*, which is closed under finite intersections and (finite or infinite) unions. The fundamental concepts of topology, such as * continuity *, * compactness *, and * connectedness *, can be defined in terms of open sets. Intuitively, continuous functions take nearby points to nearby points. Compact sets are those that can be covered by finitely many sets of arbitrarily small size. Connected sets are sets that cannot be divided into two pieces that are far apart. The words *nearby*, *arbitrarily small*, and *far apart* can all be made precise by using open sets. Several topologies can be defined on a given space. Changing a topology consists of changing the collection of open sets. This changes which functions are continuous and which subsets are compact or connected.

Metric spaces are an important class of topological spaces where the distance between any two points is defined by a function called a *metric*. In a metric space, an open set is a union of open disks, where an open disk of radius r centered at x is the set of all points whose distance to x is less than r. Many common spaces are topological spaces whose topology can be defined by a metric. This is the case of the real line, the complex plane, real and complex vector spaces and Euclidean spaces. Having a metric simplifies many proofs.

Algebraic topology is a branch of mathematics that uses tools from algebra to study topological spaces.^{ [13] } The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify up to homotopy equivalence.

The most important of these invariants are homotopy groups, homology, and cohomology.

Although algebraic topology primarily uses algebra to study topological problems, using topology to solve algebraic problems is sometimes also possible. Algebraic topology, for example, allows for a convenient proof that any subgroup of a free group is again a free group.

Differential topology is the field dealing with differentiable functions on differentiable manifolds.^{ [14] } It is closely related to differential geometry and together they make up the geometric theory of differentiable manifolds.

More specifically, differential topology considers the properties and structures that require only a smooth structure on a manifold to be defined. Smooth manifolds are "softer" than manifolds with extra geometric structures, which can act as obstructions to certain types of equivalences and deformations that exist in differential topology. For instance, volume and Riemannian curvature are invariants that can distinguish different geometric structures on the same smooth manifold –that is, one can smoothly "flatten out" certain manifolds, but it might require distorting the space and affecting the curvature or volume.

Geometric topology is a branch of topology that primarily focuses on low-dimensional manifolds (that is, spaces of dimensions 2, 3, and 4) and their interaction with geometry, but it also includes some higher-dimensional topology.^{ [15] } Some examples of topics in geometric topology are orientability, handle decompositions, local flatness, crumpling and the planar and higher-dimensional Schönflies theorem.

In high-dimensional topology, characteristic classes are a basic invariant, and surgery theory is a key theory.

Low-dimensional topology is strongly geometric, as reflected in the uniformization theorem in 2 dimensions – every surface admits a constant curvature metric; geometrically, it has one of 3 possible geometries: positive curvature/spherical, zero curvature/flat, and negative curvature/hyperbolic – and the geometrization conjecture (now theorem) in 3 dimensions – every 3-manifold can be cut into pieces, each of which has one of eight possible geometries.

2-dimensional topology can be studied as complex geometry in one variable (Riemann surfaces are complex curves) – by the uniformization theorem every conformal class of metrics is equivalent to a unique complex one, and 4-dimensional topology can be studied from the point of view of complex geometry in two variables (complex surfaces), though not every 4-manifold admits a complex structure.

Occasionally, one needs to use the tools of topology but a "set of points" is not available. In pointless topology one considers instead the lattice of open sets as the basic notion of the theory,^{ [16] } while Grothendieck topologies are structures defined on arbitrary categories that allow the definition of sheaves on those categories, and with that the definition of general cohomology theories.^{ [17] }

Topology has been used to study various biological systems including molecules and nanostructure (e.g., membraneous objects). In particular, circuit topology and knot theory have been extensively applied to classify and compare the topology of folded proteins and nucleic acids. Circuit topology classifies folded molecular chains based on the pairwise arrangement of their intra-chain contacts and chain crossings. Knot theory, a branch of topology, is used in biology to study the effects of certain enzymes on DNA. These enzymes cut, twist, and reconnect the DNA, causing knotting with observable effects such as slower electrophoresis.^{ [18] }

Topological data analysis uses techniques from algebraic topology to determine the large scale structure of a set (for instance, determining if a cloud of points is spherical or toroidal). The main method used by topological data analysis is to:

- Replace a set of data points with a family of simplicial complexes, indexed by a proximity parameter.
- Analyse these topological complexes via algebraic topology – specifically, via the theory of persistent homology.
^{ [19] } - Encode the persistent homology of a data set in the form of a parameterized version of a Betti number, which is called a barcode.
^{ [19] }

Several branches of programming language semantics, such as domain theory, are formalized using topology. In this context, Steve Vickers, building on work by Samson Abramsky and Michael B. Smyth, characterizes topological spaces as Boolean or Heyting algebras over open sets, which are characterized as semidecidable (equivalently, finitely observable) properties.^{ [20] }

Topology is relevant to physics in areas such as condensed matter physics,^{ [21] } quantum field theory and physical cosmology.

The topological dependence of mechanical properties in solids is of interest in disciplines of mechanical engineering and materials science. Electrical and mechanical properties depend on the arrangement and network structures of molecules and elementary units in materials.^{ [22] } The compressive strength of crumpled topologies is studied in attempts to understand the high strength to weight of such structures that are mostly empty space.^{ [23] } Topology is of further significance in Contact mechanics where the dependence of stiffness and friction on the dimensionality of surface structures is the subject of interest with applications in multi-body physics.

A topological quantum field theory (or topological field theory or TQFT) is a quantum field theory that computes topological invariants.

Although TQFTs were invented by physicists, they are also of mathematical interest, being related to, among other things, knot theory, the theory of four-manifolds in algebraic topology, and to the theory of moduli spaces in algebraic geometry. Donaldson, Jones, Witten, and Kontsevich have all won Fields Medals for work related to topological field theory.

The topological classification of Calabi–Yau manifolds has important implications in string theory, as different manifolds can sustain different kinds of strings.^{ [24] }

In cosmology, topology can be used to describe the overall shape of the universe.^{ [25] } This area of research is commonly known as spacetime topology.

In condensed matter a relevant application to topological physics comes from the possibility to obtain one-way current, which is a current protected from backscattering. It was first discovered in electronics with the famous quantum Hall effect, and then generalized in other areas of physics, for instance in photonics^{ [26] } by F.D.M Haldane.

The possible positions of a robot can be described by a manifold called configuration space.^{ [27] } In the area of motion planning, one finds paths between two points in configuration space. These paths represent a motion of the robot's joints and other parts into the desired pose.^{ [28] }

Disentanglement puzzles are based on topological aspects of the puzzle's shapes and components.^{ [29] }^{ [30] }^{ [31] }

In order to create a continuous join of pieces in a modular construction, it is necessary to create an unbroken path in an order which surrounds each piece and traverses each edge only once. This process is an application of the Eulerian path.^{ [32] }

*Geometry & Topology-*a mathematic research journal focused on geometry and topology, and their applications, published by Mathematical Sciences Publishers.*Journal of Topology-*a scientific journal which publishes papers of high quality and significance in topology, geometry, and adjacent areas of mathematics.

- Munkres, James R. (2000).
*Topology*(2nd ed.). Upper Saddle River, NJ: Prentice Hall. ISBN 978-0-13-181629-9 - Willard, Stephen (2016).
*General topology*. Dover books on mathematics. Mineola, N.Y: Dover publications. ISBN 978-0-486-43479-7 - Armstrong, M. A. (1983).
*Basic topology*. Undergraduate texts in mathematics. New York: Springer-Verlag. ISBN 978-0-387-90839-7

- Characterizations of the category of topological spaces
- Equivariant topology
- List of algebraic topology topics
- List of examples in general topology
- List of general topology topics
- List of geometric topology topics
- List of topology topics
- Publications in topology
- Topoisomer
- Topology glossary
- Topological Galois theory
- Topological geometry
- Topological order

In mathematics, **differential topology** is the field dealing with the topological properties and smooth properties of smooth manifolds. In this sense differential topology is distinct from the closely related field of differential geometry, which concerns the *geometric* properties of smooth manifolds, including notions of size, distance, and rigid shape. By comparison differential topology is concerned with coarser properties, such as the number of holes in a manifold, its homotopy type, or the structure of its diffeomorphism group. Because many of these coarser properties may be captured algebraically, differential topology has strong links to algebraic topology.

In mathematics and more specifically in topology, a **homeomorphism**, also called **topological isomorphism**, or **bicontinuous function**, is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isomorphisms in the category of topological spaces—that is, they are the mappings that preserve all the topological properties of a given space. Two spaces with a homeomorphism between them are called **homeomorphic**, and from a topological viewpoint they are the same.

In topology and related areas of mathematics, a **metrizable space** is a topological space that is homeomorphic to a metric space. That is, a topological space is said to be metrizable if there is a metric such that the topology induced by is **Metrization theorems** are theorems that give sufficient conditions for a topological space to be metrizable.

In the part of mathematics referred to as topology, a **surface** is a two-dimensional manifold. Some surfaces arise as the boundaries of three-dimensional solid figures; for example, the sphere is the boundary of the solid ball. Other surfaces arise as graphs of functions of two variables; see the figure at right. However, surfaces can also be defined abstractly, without reference to any ambient space. For example, the Klein bottle is a surface that cannot be embedded in three-dimensional Euclidean space.

In mathematics, a **topological space** is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called points, along with an additional structure called a topology, which can be defined as a set of neighbourhoods for each point that satisfy some axioms formalizing the concept of closeness. There are several equivalent definitions of a topology, the most commonly used of which is the definition through open sets, which is easier than the others to manipulate.

This is a glossary of some terms used in the branch of mathematics known as topology. Although there is no absolute distinction between different areas of topology, the focus here is on general topology. The following definitions are also fundamental to algebraic topology, differential topology and geometric topology. For a list of terms specific to algebraic topology, see Glossary of algebraic topology.

**Algebraic topology** is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify up to homotopy equivalence.

In mathematics, particularly in complex analysis, a **Riemann surface** is a connected one-dimensional complex manifold. These surfaces were first studied by and are named after Bernhard Riemann. Riemann surfaces can be thought of as deformed versions of the complex plane: locally near every point they look like patches of the complex plane, but the global topology can be quite different. For example, they can look like a sphere or a torus or several sheets glued together.

In mathematics, **general topology** is the branch of topology that deals with the basic set-theoretic definitions and constructions used in topology. It is the foundation of most other branches of topology, including differential topology, geometric topology, and algebraic topology.

In mathematics, **geometric topology** is the study of manifolds and maps between them, particularly embeddings of one manifold into another.

In mathematics, **low-dimensional topology** is the branch of topology that studies manifolds, or more generally topological spaces, of four or fewer dimensions. Representative topics are the structure theory of 3-manifolds and 4-manifolds, knot theory, and braid groups. This can be regarded as a part of geometric topology. It may also be used to refer to the study of topological spaces of dimension 1, though this is more typically considered part of continuum theory.

In mathematics, in the subfield of geometric topology, the **mapping class group** is an important algebraic invariant of a topological space. Briefly, the mapping class group is a certain discrete group corresponding to symmetries of the space.

In mathematics, **triangulation** describes the replacement of topological spaces by piecewise linear spaces, i.e. the choice of a homeomorphism in a suitable simplicial complex. Spaces being homeomorphic to a simplicial complex are called triangulable. Triangulation has various uses in different branches of mathematics, for instance in algebraic topology, in complex analysis or in modeling.

In mathematics, a **manifold** is a topological space that locally resembles Euclidean space near each point. More precisely, an -dimensional manifold, or *-manifold* for short, is a topological space with the property that each point has a neighborhood that is homeomorphic to an open subset of -dimensional Euclidean space.

In topology, a **topological manifold** is a topological space that locally resembles real *n*-dimensional Euclidean space. Topological manifolds are an important class of topological spaces, with applications throughout mathematics. All manifolds are topological manifolds by definition. Other types of manifolds are formed by adding structure to a topological manifold. Every manifold has an "underlying" topological manifold, obtained by simply "forgetting" the added structure. However, not every topological manifold can be endowed with a particular additional structure. For example, the E8 manifold is a topological manifold which cannot be endowed with a differentiable structure.

In mathematics, a **Hilbert manifold** is a manifold modeled on Hilbert spaces. Thus it is a separable Hausdorff space in which each point has a neighbourhood homeomorphic to an infinite dimensional Hilbert space. The concept of a Hilbert manifold provides a possibility of extending the theory of manifolds to infinite-dimensional setting. Analogous to the finite-dimensional situation, one can define a *differentiable* Hilbert manifold by considering a maximal atlas in which the transition maps are differentiable.

In mathematics, a **space** is a set endowed with a structure defining the relationships among the elements of the set. A **subspace** is a subset of the parent space which retains the same structure. While modern mathematics uses many types of spaces, such as Euclidean spaces, linear spaces, topological spaces, Hilbert spaces, or probability spaces, it does not define the notion of "space" itself.

In mathematics, specifically geometry and topology, the **classification of manifolds** is a basic question, about which much is known, and many open questions remain.

In mathematics, **geometry and topology** is an umbrella term for the historically distinct disciplines of geometry and topology, as general frameworks allow both disciplines to be manipulated uniformly, most visibly in local to global theorems in Riemannian geometry, and results like the Gauss–Bonnet theorem and Chern–Weil theory.

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*Differential Equations: A Dynamical Systems Approach. Part II: Higher-Dimensional Systems*. Texts in Applied Mathematics. Vol. 18. Springer. p. 204. ISBN 978-0-387-94377-0. - 1 2 Croom 1989 , p. 7
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*The Knot Book: An Elementary Introduction to the Mathematical Theory of Knots*. American Mathematical Society. ISBN 978-0-8218-3678-1. - 1 2 Gunnar Carlsson (April 2009). "Topology and data" (PDF).
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*Topology via Logic*. Cambridge Tracts in Theoretical Computer Science. Cambridge University Press. ISBN 978-0521576512. - ↑ "The Nobel Prize in Physics 2016". Nobel Foundation. 4 October 2016. Retrieved 12 October 2016.
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*The Shape of Space: How to Visualize Surfaces and Three-dimensional Manifolds*2nd ed (Marcel Dekker, 1985, ISBN 0-8247-7437-X) - ↑ Haldane, F. D. M.; Raghu, S. (10 January 2008). "Possible Realization of Directional Optical Waveguides in Photonic Crystals with Broken Time-Reversal Symmetry".
*Physical Review Letters*.**100**(1): 013904. arXiv: cond-mat/0503588 . Bibcode:2008PhRvL.100a3904H. doi:10.1103/PhysRevLett.100.013904. ISSN 0031-9007. PMID 18232766. S2CID 44745453. - ↑ John J. Craig,
*Introduction to Robotics: Mechanics and Control*, 3rd Ed. Prentice-Hall, 2004 - ↑ Farber, Michael (2008).
*Invitation to Topological Robotics*. European Mathematical Society. ISBN 978-3037190548. - ↑ Horak, Mathew (2006). "Disentangling Topological Puzzles by Using Knot Theory".
*Mathematics Magazine*.**79**(5): 368–375. doi:10.2307/27642974. JSTOR 27642974. - ↑ http://sma.epfl.ch/Notes.pdf Archived 1 November 2022 at the Wayback Machine A Topological Puzzle, Inta Bertuccioni, December 2003.
- ↑ https://www.futilitycloset.com/the-figure-8-puzzle Archived 25 May 2017 at the Wayback Machine The Figure Eight Puzzle, Science and Math, June 2012.
- ↑ Eckman, Edie (2012).
*Connect the shapes crochet motifs: creative techniques for joining motifs of all shapes*. Storey Publishing. ISBN 978-1603429733.

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*Topology and Groupoids*. Booksurge. ISBN 978-1-4196-2722-4. (Provides a well motivated, geometric account of general topology, and shows the use of groupoids in discussing van Kampen's theorem, covering spaces, and orbit spaces.) - Wacław Sierpiński,
*General Topology*, Dover Publications, 2000, ISBN 0-486-41148-6 - Pickover, Clifford A. (2006).
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*Elementary Topology*(2nd ed.). Dover Publications Inc. ISBN 978-0-486-66522-1.

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Wikibooks has more on the topic of: ** Topology **

- "Topology, general".
*Encyclopedia of Mathematics*. EMS Press. 2001 [1994]. - Elementary Topology: A First Course Viro, Ivanov, Netsvetaev, Kharlamov.
- The Topological Zoo at The Geometry Center.
- Topology Atlas
- Topology Course Lecture Notes Aisling McCluskey and Brian McMaster, Topology Atlas.
- Topology Glossary
- Moscow 1935: Topology moving towards America, a historical essay by Hassler Whitney.

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