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| [[Image:Simplicial complex example.svg|thumb|200px|A simplicial 3-complex.]] | | Gabrielle is what her his [http://mondediplo.com/spip.php?page=recherche&recherche=conversation+loves conversation loves] to call lady's though she doesn't genuinely like being called in that way. As a woman what your girl really likes is mah jongg but she doesn't have made a dime utilizing it. Software developing is where her center income comes from truthfully soon her husband as well as a her will start specific own business. For a while she's yet been in Massachusetts. Go to her [http://www.dailymail.co.uk/home/search.html?sel=site&searchPhrase=website website] to come out more: http://prometeu.net<br><br> |
| [[Image:Simplicial complex nonexample.svg|thumb|200px|An arrangement of simplices that is not a valid simplicial complex.]]
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| In [[mathematics]], a '''simplicial complex''' is a [[topological space]] of a certain kind, constructed by "gluing together" [[Point (geometry)|point]]s, [[line segment]]s, [[triangle]]s, and their [[Simplex|''n''-dimensional counterparts]] (see illustration). Simplicial complexes should not be confused with the more abstract notion of a [[simplicial set]] appearing in modern simplicial homotopy theory. The purely combinatorial counterpart to a simplicial complex is an [[abstract simplicial complex]].
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| ==Definitions==
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| A '''simplicial complex''' <math>\mathcal{K}</math> is a set of [[simplices]] that satisfies the following conditions:
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| :1. Any [[Simplex#Elements|face]] of a simplex from <math>\mathcal{K}</math> is also in <math>\mathcal{K}</math>.
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| :2. The [[Set intersection|intersection]] of any two simplices <math>\sigma_1, \sigma_2 \in \mathcal{K}</math> is a face of both <math>\sigma_1</math> and <math>\sigma_2</math>.
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| Note that the empty set is a face of every simplex. See also the definition of an [[abstract simplicial complex]], which loosely speaking is a simplicial complex without an associated geometry.
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| A '''simplicial ''k''-complex''' <math>\mathcal{K}</math> is a simplicial complex where the largest dimension of any simplex in <math>\mathcal{K}</math> equals ''k''. For instance, a simplicial 2-complex must contain at least one triangle, and must not contain any tetrahedra or higher-dimensional simplices.
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| A '''pure''' or '''homogeneous''' simplicial ''k''-complex <math>\mathcal{K}</math> is a simplicial complex where every simplex of dimension less than ''k'' is a face of some simplex <math>\sigma \in \mathcal{K}</math> of dimension exactly ''k''. Informally, a pure 1-complex "looks" like it's made of a bunch of lines, a 2-complex "looks" like it's made of a bunch of triangles, etc. An example of a ''non''-homogeneous complex is a triangle with a line segment attached to one of its vertices.
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| A '''facet''' is any simplex in a complex that is ''not'' a face of any larger simplex. (Note the difference from a "face" of a simplex). A pure simplicial complex can be thought of as a complex where all facets have the same dimension.
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| Sometimes the term ''face'' is used to refer to a simplex of a complex, not to be confused with a face of a simplex.
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| For a simplicial complex embedded in a ''k''-dimensional space, the ''k''-faces are sometimes referred to as its '''cells'''. The term ''cell'' is sometimes used in a broader sense to denote a set [[Homeomorphism|homeomorphic]] to a simplex, leading to the definition of [[cell complex]].
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| The '''underlying space''', sometimes called the '''carrier''' of a simplicial complex is the [[union (set theory)|union]] of its simplices.
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| ==Closure, star, and link==
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| <table>
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| <tr>
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| <td>[[Image:Simplicial complex closure.png|thumb|350px|Two <span style="color:#B99A01;">simplices</span> and their <span style="color:#40864B;">'''closure'''</span>.]]</td>
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| </tr>
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| <tr>
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| <td>[[Image:Simplicial complex star.png|thumb|350px|A <span style="color:#B99A01;">simplex</span> and its <span style="color:#40864B;">'''star'''</span>.]]</td>
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| <td>[[Image:Simplicial complex link.png|thumb|350px|A <span style="color:#B99A01;">simplex</span> and its <span style="color:#40864B;">'''link'''</span>.]]</td>
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| </tr>
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| </table>
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| Let ''K'' be a simplicial complex and let ''S'' be a collection of simplices in ''K''.
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| The '''closure''' of ''S'' (denoted Cl ''S'') is the smallest simplicial subcomplex of ''K'' that contains
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| each simplex in ''S''. Cl ''S'' is obtained by repeatedly adding to ''S'' each face of every simplex in ''S''.
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| The '''star''' of ''S'' (denoted St ''S'') is the set of all simplices in ''K'' that have any faces in ''S''. (Note that the star is generally not a simplicial complex itself).
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| The '''link''' of ''S'' (denoted Lk ''S'') equals Cl St ''S'' - St Cl ''S''.
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| It is the closed star of ''S'' minus the stars of all faces of ''S''.
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| ==Algebraic topology==
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| In [[algebraic topology]] simplicial complexes are often useful for concrete calculations. For the definition of [[homology group]]s of a simplicial complex, one can read the corresponding [[chain complex]] directly, provided that consistent orientations are made of all simplices. The requirements of [[homotopy theory]] lead to the use of more general spaces, the [[CW complex]]es. Infinite complexes are a technical tool basic in [[algebraic topology]]. See also the discussion at [[polytope]] of simplicial complexes as subspaces of Euclidean space, made up of subsets each of which is a [[simplex]]. That somewhat more concrete concept is there attributed to [[Pavel Sergeevich Alexandrov|Alexandrov]]. Any finite simplicial complex in the sense talked about here can be embedded as a polytope in that sense, in some large number of dimensions. In algebraic topology a compact topological space which is homeomorphic to the geometric realization of a finite simplicial complex is usually called a [[polyhedron]] (see {{harvnb|Spanier|1966}}, {{harvnb|Maunder|1996}}, {{harvnb|Hilton|Wylie|1967}}).
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| ==Combinatorics==
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| [[Combinatorics|Combinatorialists]] often study the '''f-vector''' of a simplicial d-complex Δ, which is the [[integer|integral]] sequence <math>(f_0, f_1, f_2, ..., f_{d+1})</math>, where ''f<sub>i</sub>'' is the number of (''i''−1)-dimensional faces of Δ (by convention, ''f''<sub>0</sub> = 1 unless Δ is the empty complex). For instance, if Δ is the boundary of the [[octahedron]], then its f-vector is (1, 6, 12, 8), and if Δ is the first simplicial complex pictured above, its ''f''-vector is (1, 18, 23, 8, 1). A complete characterization of the possible f-vectors of simplicial complexes is given by the [[Kruskal–Katona theorem|Kruskal-Katona theorem]].
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| By using the f-vector of a simplicial d-complex Δ as coefficients of a [[polynomial]] (written in decreasing order of exponents), we obtain the '''f-polynomial''' of Δ. In our two examples above, the ''f''-polynomials would be <math>x^3+6x^2+12x+8</math> and <math>x^4+18x^3+23x^2+8x+1</math>, respectively.
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| Combinatorists are often quite interested in the '''h-vector''' of a simplicial complex Δ, which is the sequence of coefficients of the polynomial that results from plugging ''x''−1 into the f-polynomial of Δ. Formally, if we write ''F''<sub>Δ</sub>(''x'') to mean the ''f''-polynomial of Δ, then the '''h-polynomial''' of Δ is
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| :<math>F_\Delta(x-1)=h_0x^{d+1}+h_1x^d+h_2x^{d-1}+...+h_dx+h_{d+1}</math> | |
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| and the h-vector of Δ is
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| :<math>(h_0, h_1, h_2, ..., h_{d+1}).</math>
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| We calculate the h-vector of the octahedron boundary (our first example) as follows:
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| :<math>F(x-1)=(x-1)^3+6(x-1)^2+12(x-1)+8=x^3+3x^2+3x+1.</math>
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| So the h-vector of the boundary of the octahedron is (1, 3, 3, 1). It is not an accident this h-vector is symmetric. In fact, this happens whenever Δ is the boundary of a simplicial [[polytope]] (these are the [[Dehn-Sommerville equations]]). In general, however, the h-vector of a simplicial complex is not even necessarily positive. For instance, if we take Δ to be the 2-complex given by two triangles intersecting only at a common vertex, the resulting h-vector is (1, 3, −2).
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| A complete characterization of all simplicial polytope h-vectors is given by the celebrated [[g-theorem]] of [[Richard P. Stanley|Stanley]], Billera, and Lee.
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| Simplicial complexes can be seen to have the same geometric structure as the contact graph of a sphere packing (a graph where vertices are the centers of spheres and edges exist if the corresponding packing elements touch each other) and as such can be used to determine the combinatorics of [[sphere packing]]s, such as the number of touching pairs (1-simplices), touching triplets (2-simplices), and touching quadruples (3-simplices) in a sphere packing.
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| ==See also==
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| * [[Abstract simplicial complex]]
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| * [[Barycentric subdivision]]
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| * [[Causal dynamical triangulation]]
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| * [[Polygonal chain]]{{spaced ndash}} 1 dimensional simplicial complex
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| ==References==
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| *{{citation|last=Spanier|first=E.H.|title=Algebraic Topology|year=1966|publisher=Springer|isbn=0-387-94426-5}}
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| *{{citation|last=Maunder|first=C.R.F.|title=Algebraic Topology|year=1996|publisher=Dover|isbn=0-486-69131-4}}
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| *{{citation|last=Hilton|first=P.J.|last2=Wylie|first2=S.|title=Homology Theory|year=1967|publisher=Cambridge University Press|isbn=0-521-09422-4}}
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| ==External links==
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| * {{mathworld | urlname = SimplicialComplex | title = Simplicial complex}}
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| [[Category:Topological spaces]]
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| [[Category:Algebraic topology]]
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| [[Category:Simplicial sets]]
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| [[Category:Triangulation (geometry)]]
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