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In [[mathematics]], and more specifically in [[algebraic topology]] and [[polyhedral combinatorics]], the '''Euler characteristic''' (or '''Euler&ndash;Poincaré characteristic''') is a [[topological invariant]], a number that describes a [[topological space]]'s shape or structure regardless of the way it is bent. It is commonly denoted by <math>\chi</math> ([[Greek alphabet|Greek letter]] [[chi (letter)|chi]]).

The Euler characteristic was originally defined for [[polyhedron|polyhedra]] and used to prove various theorems about them, including the classification of the [[Platonic solid]]s. [[Leonhard Euler]], for whom the concept is named, was responsible for much of this early work. In modern mathematics, the Euler characteristic arises from [[homology (mathematics)|homology]] and, more abstractly, [[homological algebra]].

== Polyhedra ==
The '''Euler characteristic''' <math>\chi</math> was classically defined for the surfaces of polyhedra, according to the formula

:<math>\chi=V-E+F \,\!</math>

where ''V'', ''E'', and ''F'' are respectively the numbers of [[Vertex (geometry)|vertices]] (corners), [[Edge (geometry)|edge]]s and [[Face (geometry)|faces]] in the given polyhedron. Any [[convex polyhedron]]'s surface has Euler characteristic

:<math>\chi = V - E + F = 2. \,\!</math>

This result is known as '''Euler's polyhedron formula''' or '''theorem'''. It corresponds to the Euler characteristic of the [[sphere]] (i.e. χ = 2), and applies identically to [[spherical polyhedra]]. An illustration of the formula on some polyhedra is given below.

{| class="wikitable"
|-
!Name
!Image
!Vertices<br>''V''
!Edges<br>''E''
!Faces<br>''F''
!Euler characteristic:<br>''V'' &minus; ''E'' + ''F''
|- align=center
|[[Tetrahedron]]
|[[Image:tetrahedron.png|50px]]
|4
|6
|4
|'''2'''
|- align=center
|[[Hexahedron]] or [[cube (geometry)|cube]]
|[[Image:hexahedron.png|50px]]
|8
|12
|6
|'''2'''
|- align=center
|[[Octahedron]]
|[[Image:octahedron.png|50px]]
|6
|12
|8
|'''2'''
|- align=center
|[[Dodecahedron]]
|[[Image:dodecahedron.png|50px]]
|20
|30
|12
|'''2'''
|- align=center
|[[Icosahedron]]
|[[Image:icosahedron.png|50px]]
|12
|30
|20
|'''2'''
|}

The surfaces of nonconvex polyhedra can have various Euler characteristics;

{| class="wikitable"
|-
!Name
!Image
!Vertices<br>''V''
!Edges<br>''E''
!Faces<br>''F''
!Euler characteristic:<br>''V'' &minus; ''E'' + ''F''
|- align=center
|[[Tetrahemihexahedron]]
|[[Image:Tetrahemihexahedron.png|100px]]
|6
|12
|7
|'''1'''
|- align=center
|[[Octahemioctahedron]]
|[[Image:Octahemioctahedron.png|100px]]
|12
|24
|12
|'''0'''
|- align=center
|[[Cubohemioctahedron]]
|[[Image:Cubohemioctahedron.png|100px]]
|12
|24
|10
|'''&minus;2'''
|- align=center
|[[Great icosahedron]]
|[[Image:Great icosahedron.png|100px]]
|12
|30
|20
|'''2'''
|}

For regular polyhedra, [[Arthur Cayley]] derived a modified form of Euler's formula using the [[Density (polytope)|density]] ''D'', [[vertex figure]] ''d''<sub>''v''</sub>, and faces <math>d_f</math>:
:<math>d_v V - E + d_f F = 2 D.</math>
This version holds both for convex polyhedra (where the densities are all 1) and the non-convex [[Kepler–Poinsot polyhedron|Kepler-Poinsot polyhedra]].

[[Projective polyhedra]] all have Euler characteristic 1, like the [[real projective plane]], while the surfaces of [[toroidal polyhedra]] all have Euler characteristic 0, like the [[torus]].

===Planar graphs===
{{See also|Planar graph#Euler's formula}}
The Euler characteristic can be defined for connected [[planar graph]]s by the same <math>V - E + F</math> formula as for polyhedral surfaces, where ''F'' is the number of faces in the graph, including the exterior face.

The Euler characteristic of any planar connected graph G is 2. This is easily proved by induction on the number of faces determined by G, starting with a tree as the base case. For trees, E = V-1 and F = 1. If G has C components, the same argument by induction on F shows that <math>V - E + F - C = 1</math>. One of the few graph theory papers of Cauchy also proves this result.

Via [[stereographic projection]] the plane maps to the two-dimensional sphere, such that a connected graph maps to a polygonal decomposition of the sphere, which has Euler characteristic 2. This viewpoint is implicit in Cauchy's proof of Euler's formula given below.

===Proof of Euler's formula===
[[Image:V-E+F=2 Proof Illustration.svg|frame|right|First steps of the proof in the case of a cube]]

There are many proofs of Euler's formula. One was given by [[Augustin Louis Cauchy|Cauchy]] in 1811, as follows. It applies to any convex polyhedron, and more generally to any polyhedron whose boundary is topologically equivalent to a sphere and whose faces are topologically equivalent to disks.

Remove one face of the polyhedral surface. By pulling the edges of the missing face away from each other, deform all the rest into a planar graph of points and curves, as illustrated by the first of the three graphs for the special case of the cube. (The assumption that the polyhedral surface is homeomorphic to the sphere at the beginning is what makes this possible.) After this deformation, the regular faces are generally not regular anymore. The number of vertices and edges has remained the same, but the number of faces has been reduced by 1.  Therefore, proving Euler's formula for the polyhedron reduces to proving ''V'' &minus; ''E'' + ''F'' =1 for this deformed, planar object.

If there is a face with more than three sides, draw a diagonal—that is, a curve through the face connecting two vertices that aren't connected yet. This adds one edge and one face and does not change the number of vertices, so it does not change the quantity ''V'' &minus; ''E'' + ''F''. (The assumption that all faces are disks is needed here, to show via the [[Jordan curve theorem]] that this operation increases the number of faces by one.) Continue adding edges in this manner until all of the faces are triangular.

Apply repeatedly either of the following two transformations, maintaining the invariant that the exterior boundary is always a [[simple cycle]]:
#Remove a triangle with only one edge adjacent to the exterior, as illustrated by the second graph. This decreases the number of edges and faces by one each and does not change the number of vertices, so it preserves ''V'' &minus; ''E'' + ''F''.
#Remove a triangle with two edges shared by the exterior of the network, as illustrated by the third graph. Each triangle removal removes a vertex, two edges and one face, so it preserves ''V'' &minus; ''E'' + ''F''.
These transformations eventually reduce the planar graph to a single triangle.  (Without the simple-cycle invariant, removing a triangle might disconnect the remaining triangles, invalidating the rest of the argument. A valid removal order is an elementary example of a [[shelling (topology)|shelling]].)

At this point the lone triangle has ''V'' = 3, ''E'' = 3, and ''F'' = 1, so that ''V'' &minus; ''E'' + ''F'' = 1.  Since each of the two above transformation steps preserved this quantity, we have shown ''V'' &minus; ''E'' + ''F'' = 1 for the deformed, planar object thus demonstrating ''V'' &minus; ''E'' + ''F'' = 2 for the polyhedron. This proves the theorem.

For additional proofs, see ''Twenty Proofs of Euler's Formula'' by [[David Eppstein]].<ref name=Eppstein2013>{{cite web|last=Eppstein|first=David|title=Twenty Proofs of Euler's Formula: V-E+F=2|url=http://www.ics.uci.edu/~eppstein/junkyard/euler/|accessdate=3 June 2013}}</ref> Multiple proofs, including their flaws and limitations, are used as examples in ''[[Proofs and Refutations]]'' by [[Imre Lakatos]].<ref>[[Imre Lakatos]]: [[Proofs and Refutations]], Cambridge Technology Press, 1976</ref>

==Topological definition==
The polyhedral surfaces discussed above are, in modern language, two-dimensional finite [[CW-complex]]es. (When only triangular faces are used, they are two-dimensional finite [[simplicial complex]]es.) In general, for any finite CW-complex, the '''Euler characteristic''' can be defined as the alternating sum

:<math>\chi = k_0 - k_1 + k_2 - k_3 + \cdots,\ </math>

where ''k''<sub>''n''</sub> denotes the number of cells of dimension ''n'' in the complex.

Similarly, for a simplicial complex, the '''Euler characteristic''' equals the alternating sum

:<math>\chi = k_0 - k_1 + k_2 - k_3 + \cdots,\ </math>

where ''k''<sub>''n''</sub> denotes the number of ''n''-simplexes in the complex.

More generally still, for any [[topological space]], we can define the ''n''th [[Betti number]] ''b''<sub>''n''</sub> as the [[rank of an abelian group|rank]] of the ''n''-th [[singular homology]] group. The '''Euler characteristic''' can then be defined as the alternating sum

:<math>\chi = b_0 - b_1 + b_2 - b_3 + \cdots.\ </math>

This quantity is well-defined if the Betti numbers are all finite and if they are zero beyond a certain index&nbsp;''n''<sub>0</sub>. For simplicial complexes, this is not the same definition as in the previous paragraph but a homology computation shows that the two definitions will give the same value for <math>\chi</math>.

==Properties==

The Euler characteristic behaves well with respect to many basic operations on topological spaces, as follows.

===Homotopy invariance===
Homology is a topological invariant, and moreover a [[homotopy invariant]]: Two topological spaces that are [[homotopy equivalent]] have [[group isomorphism|isomorphic]] homology groups. It follows that the Euler characteristic is also a homotopy invariant.

For example, any [[contractible]] space (that is, one homotopy equivalent to a point) has trivial homology, meaning that the 0th Betti number is 1 and the others 0. Therefore its Euler characteristic is 1. This case includes [[Euclidean space]] <math>\mathbb{R}^n</math> of any dimension, as well as the solid unit ball in any Euclidean space &mdash; the one-dimensional interval, the two-dimensional disk, the three-dimensional ball, etc.

For another example, any convex polyhedron is homeomorphic to the three-dimensional [[ball (mathematics)|ball]], so its surface is homeomorphic (hence homotopy equivalent) to the two-dimensional [[sphere]], which has Euler characteristic&nbsp;2. This explains why convex polyhedra have Euler characteristic 2.

===Inclusion-exclusion principle===
If ''M'' and ''N'' are any two topological spaces, then the Euler characteristic of their [[disjoint union]] is the sum of their Euler characteristics, since homology is additive under disjoint union:

:<math>\chi(M \sqcup N) = \chi(M) + \chi(N).</math>

More generally, if ''M'' and ''N'' are subspaces of a larger space ''X'', then so are their union and intersection. In some cases, the Euler characteristic obeys a version of the [[inclusion-exclusion principle]]:

:<math>\chi(M \cup N) = \chi(M) + \chi(N) - \chi(M \cap N).</math>

This is true in the following cases:

*if ''M'' and ''N'' are an [[excisive couple]]. In particular, if the [[interior (topology)|interiors]] of ''M'' and ''N'' inside the union still cover the union.<ref>Edwin Spanier: Algebraic Topology, Springer 1966, p. 205.</ref>

*if ''X'' is a [[locally compact space]], and one uses Euler characteristics with [[compact space|compact]] [[support (mathematics)|supports]], no assumptions on ''M'' or ''N'' are needed.

*if ''X'' is a [[topologically stratified space|stratified space]] all of whose strata are even dimensional, the inclusion-exclusion principle holds if ''M'' and ''N'' are unions of strata.  This applies in particular if ''M'' and ''N'' are subvarieties of a [[complex number|complex]] [[algebraic variety]].<ref>William Fulton: Introduction to toric varieties, 1993, Princeton University Press, p. 141.</ref>

In general, the inclusion-exclusion principle is false. A [[counterexample]] is given by taking ''X'' to be the [[real line]], ''M'' a [[subset]] consisting of one point and ''N'' the [[complement (set theory)|complement]] of ''M''.

===Product property===
Also, the Euler characteristic of any [[product space]] ''M'' &times; ''N'' is

:<math>\chi(M \times N) = \chi(M) \cdot \chi(N).</math>

These addition and multiplication properties are also enjoyed by [[cardinality]] of [[set (mathematics)|set]]s. In this way, the Euler characteristic can be viewed as a generalisation of cardinality; see [http://math.ucr.edu/home/baez/counting/].

===Covering spaces===
{{details|Riemann–Hurwitz formula}}
Similarly, for an ''k''-sheeted [[covering space]] <math>\tilde{M} \to M,</math> one has
:<math>\chi(\tilde{M}) = k \cdot \chi(M).</math>
More generally, for a [[ramified covering space]], the Euler characteristic of the cover can be computed from the above, with a correction factor for the ramification points, which yields the [[Riemann–Hurwitz formula]].

===Fibration property===
The product property holds much more generally, for [[fibrations]] with certain conditions.

If <math>p\colon E \to B</math> is a fibration with fiber ''F,'' with the base ''B'' [[path-connected]], and the fibration is orientable over a field ''K,'' then the Euler characteristic with coefficients in the field ''K'' satisfies the product property:<ref>{{citation
|title=Algebraic Topology
|first=Edwin Henry
|last=Spanier
|authorlink=Edwin Spanier
|publisher=Springer
|year=1982
|isbn=978-0-387-94426-5
|url=http://books.google.com/?id=h-wc3TnZMCcC
}}, [http://books.google.com/books?id=h-wc3TnZMCcC&pg=PA481 Applications of the homology spectral sequence, p. 481]</ref>
:<math>\chi(E) = \chi(F)\cdot \chi(B).</math>
This includes product spaces and covering spaces as special cases,
and can be proven by the [[Serre spectral sequence]] on homology of a fibration.

For fiber bundles, this can also be understood in terms of a [[transfer map]] <math>\tau\colon H_*(B) \to H_*(E)</math> – note that this is a lifting and goes "the wrong way" – whose composition with the projection map <math>p_*\colon H_*(E) \to H_*(B)</math> is multiplication by the [[Euler class]] of the fiber:<ref>{{citation
|title=Fibre bundles and the Euler characteristic
|first=Daniel Henry
|last=Gottlieb
|journal=Journal of Differential Geometry
|volume=10
|issue=1
|year=1975
|pages=39–48
|url=http://www.math.purdue.edu/~gottlieb/Bibliography/17FibreBundlesAndtheEulerCharacteristic.pdf
}}</ref>
:<math>p_* \circ \tau = \chi(F) \cdot 1.</math>

==Examples==

===Surfaces===
The Euler characteristic can be calculated easily for general surfaces by finding a polygonization of the surface (that is, a description as a [[CW-complex]]) and using the above definitions.
{| class="wikitable"
|-
!Name
!Image
!Euler characteristic
|-
|[[Interval (mathematics)|Interval]]
|[[Image:Complete graph K2.svg|100px]]
|'''1'''
|-
|[[Circle]]
|[[Image:Cirklo.svg|100px]]
|'''0'''
|-
|[[Unit disk|Disk]]
|[[Image:Disc Plain grey.svg|100px]]
|'''1'''
|-
|[[Sphere]]
|[[Image:Sphere-wireframe.png|100px]]
|'''2'''
|-
|[[Torus]] <br> (Product of two circles)
|[[Image:Torus illustration.png|100px]]
|'''0'''
|-
|[[Double torus]]
|[[Image:Double torus illustration.png|100px]]
|'''&minus;2'''
|-
|[[Triple torus]]
|[[Image:Triple torus illustration.png|100px]]
|'''&minus;4'''
|-
|[[Real projective plane]]
|[[Image:Steiners Roman.png|100px]]
|'''1'''
|-
|[[Möbius strip]]
|[[Image:MobiusStrip-01.png|100px]]
|'''0'''
|-
|[[Klein bottle]]
|[[Image:KleinBottle-01.png|100px]]
|'''0'''
|-
|Two spheres (not connected) <br> (Disjoint union of two spheres)
|[[Image:Sphere-wireframe.png|100px]][[Image:Sphere-wireframe.png|100px]]
|2 + 2 = '''4'''
|-
|Three spheres (not connected) <br> (Disjoint union of three spheres)
|[[Image:Sphere-wireframe.png|100px]][[Image:Sphere-wireframe.png|100px]][[Image:Sphere-wireframe.png|100px]]
|2 + 2 + 2 = '''6'''
|}

===Soccer ball===

It is common to construct [[soccer ball]]s by stitching together pentagonal and hexagonal pieces, with three pieces meeting at each vertex (see for example the [[Adidas Telstar]]). If ''P'' pentagons and ''H'' hexagons are used, then there are ''F'' = ''P'' + ''H'' faces, ''V'' = (5 ''P'' + 6 ''H'') / 3 vertices, and ''E'' = (5 ''P'' + 6 ''H'') / 2 edges. The Euler characteristic is thus
: ''V'' - ''E'' + ''F'' = (5 ''P'' + 6 ''H'') / 3 - (5 ''P'' + 6 ''H'') / 2 + ''P'' + ''H'' = ''P'' / 6.
Because the sphere has Euler characteristic 2, it follows that ''P'' = 12. That is, a soccer ball constructed in this way always has 12 pentagons. In principle, the number of hexagons is unconstrained. This result is also applicable to [[fullerene]]s.

===Arbitrary dimensions===

The ''n''-dimensional sphere has Betti number 1 in dimensions 0 and ''n'', and all other Betti numbers 0. Hence its Euler characteristic is 1 + (-1)<sup>''n''</sup> &mdash; that is, either 0 or 2.

The ''n''-dimensional real [[projective space]] is the quotient of the ''n''-sphere by the [[antipodal map]]. It follows that its Euler characteristic is exactly half that of the corresponding sphere &mdash; either 0 or 1.

The ''n''-dimensional torus is the product space of ''n'' circles. Its Euler characteristic is 0, by the product property. More generally, any [[parallelizable manifold]], including any [[Lie group]], has Euler characteristic 0.<ref>[[John Milnor|Milnor, John W.]] and Stasheff, James D.: Characteristic Classes, Princeton University Press, 1974</ref>

The Euler characteristic of any [[closed manifold|closed]] odd-dimensional manifold is also 0.<ref>Richeson, D., ''Euler's Gem'', Princeton, 2008 (p. 261)</ref> The case for orientable examples is a corollary of [[Poincaré duality]]. This property applies more generally to any [[compact space|compact]] [[topologically stratified space|stratified space]] all of whose strata have odd dimension.

== Relations to other invariants ==
The Euler characteristic of a closed [[Orientability|orientable]] [[surface]] can be calculated from its [[genus (mathematics)|genus]] ''g'' (the number of [[torus|tori]] in a [[connected sum]] decomposition of the surface; intuitively, the number of "handles") as

:<math>\chi = 2 - 2g.\ </math>

The Euler characteristic of a closed non-orientable surface can be calculated from its non-orientable genus ''k'' (the number of [[real projective plane]]s in a connected sum decomposition of the surface) as

:<math>\chi = 2 - k.\ </math>

For closed smooth manifolds, the Euler characteristic coincides with the '''Euler number''', i.e., the [[Euler class]] of its [[tangent bundle]] evaluated on the [[fundamental class]] of a manifold. The Euler class, in turn, relates to all other [[characteristic class]]es of [[vector bundle]]s.

For closed [[Riemannian manifold]]s, the Euler characteristic can also be found by integrating the curvature; see the [[Gauss&ndash;Bonnet theorem]] for the two-dimensional case and the [[generalized Gauss&ndash;Bonnet theorem]] for the general case.

A discrete analog of the Gauss&ndash;Bonnet theorem is [[René Descartes|Descartes']] theorem that the "total defect" of a [[polyhedron]], measured in full circles, is the Euler characteristic of the polyhedron; see [[defect (geometry)]].

[[Hadwiger's theorem]] characterizes the Euler characteristic as the ''unique'' ([[up to]] [[scalar multiplication]]) translation-invariant, finitely additive, not-necessarily-nonnegative set function defined on [[finite unions]] of [[compact space|compact]] [[Convex polyhedron|convex]] sets in '''R'''<sup>''n''</sup> that is "homogeneous of degree 0".

==Generalizations==
For every combinatorial [[cell complex]], one defines the Euler characteristic as the number of 0-cells, minus the number of 1-cells, plus the number of 2-cells, etc., if this alternating sum is finite. In particular, the Euler characteristic of a finite set is simply its cardinality, and the Euler characteristic of a [[graph (mathematics)|graph]] is the number of vertices minus the number of edges.<ref>Olaf Post calls this a "well-known formula": {{citation|first=Olaf|last=Post|contribution=Spectral analysis of metric graphs and related spaces|title=Limits of graphs in group theory and computer science|location=Lausanne, Switzerland|publisher=[[EPFL Press]]|year=2009|pages=109–140|arxiv=0712.1507 }}.</ref>

More generally, one can define the Euler characteristic of any [[chain complex]] to be the alternating sum of the [[rank of an abelian group|ranks]] of the homology groups of the chain complex.

A version used in [[algebraic geometry]] is as follows. For any [[Sheaf (mathematics)|sheaf]] <math>\scriptstyle\mathcal{F}</math> on a projective [[Scheme (mathematics)|scheme]] ''X'', one defines its Euler characteristic
:<math>\chi ( \mathcal{F})= \Sigma (-1)^i h^i(X,\mathcal{F}),</math>
where <math>\scriptstyle h^i(X, \mathcal{F}) </math> is the dimension of the ''i''-th [[sheaf cohomology]] group of <math>\scriptstyle\mathcal{F}</math>.

Another generalization of the concept of Euler characteristic on manifolds comes from [[orbifold]]s.  While every manifold has an integer Euler characteristic, an orbifold can have a fractional Euler characteristic.  For example, the teardrop orbifold has Euler characteristic 1&nbsp;+&nbsp;1/''p'', where ''p'' is a prime number corresponding to the cone angle 2''π''&nbsp;/&nbsp;''p''.

The concept of Euler characteristic of a bounded finite [[partially ordered set|poset]] is another generalization, important in [[combinatorics]].  A poset is "bounded" if it has smallest and largest elements; call them 0 and 1.  The Euler characteristic of such a poset is defined as the integer μ(0,1), where μ is the [[Möbius function]] in that poset's [[incidence algebra]].

This can be further generalized by defining a '''Q'''-valued Euler characteristic for certain finite [[category (mathematics)|categories]], a notion compatible with the Euler characteristics of graphs, orbifolds and posets mentioned above. In this setting, the Euler characteristic of a finite [[group (mathematics)|group]] or [[monoid]] ''G'' is 1/|''G''|, and the Euler characteristic of a finite [[groupoid]] is the sum of 1/|''G<sub>i</sub>''|, where we picked one representative group ''G<sub>i</sub>'' for each connected component of the groupoid.<ref>Tom Leinster, "[http://www.math.uiuc.edu/documenta/vol-13/02.pdf The Euler characteristic of a category]", ''Documenta Mathematica'', 13 (2008), pp. 21&ndash;49</ref>

== See also ==
* [[Euler calculus]]
* [[Euler class]]
* [[List of topics named after Leonhard Euler]]
* [[List of uniform polyhedra]]

== Notes ==
{{Reflist}}

== Further reading ==
*Richeson, David S. (2008) ''Euler's Gem: The Polyhedron Formula and the Birth of Topology''. Princeton University Press.
*H. Graham Flegg: ''From Geometry to Topology''. Dover 2001, p.&nbsp;40

== External links ==
*{{Mathworld | urlname=EulerCharacteristic | title=Euler characteristic }}
*{{Mathworld | urlname=PolyhedralFormula | title=Polyhedral formula }}
*{{SpringerEOM| title=Euler characteristic | id=Euler_characteristic | oldid=16333 | first=S.V. | last=Matveev }}

[[Category:Algebraic topology]]
[[Category:Topological graph theory]]
[[Category:Polyhedral combinatorics]]
[[Category:Articles containing proofs]]