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that is not a valid simplicial complex.]]

In Mathematics , a simplicial complex is a Topological Space of a particular kind, constructed by "gluing together" Point s, Line Segment s, Triangle s, and their ''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.


DEFINITIONS


A simplicial complex \mathcal{K} is a set of Simplices that satisfies the following conditions:
:1. Any Face of a simplex from \mathcal{K} is also in \mathcal{K}.
:2. The Intersection of any two simplices \sigma_1, \sigma_2 \in \mathcal{K} is a face of both \sigma_1 and \sigma_2.

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.

A simplicial k-complex \mathcal{K} is a simplicial complex where the largest dimension of any simplex in \mathcal{K} equals ''k''. For instance, a simplicial 2-complex must contain at least one triangle, and must not contain any tetrahedra or higher-dimension simplices.

A pure or '''homogeneous''' simplicial ''k''-complex \mathcal{K} is a simplicial complex where every simplex of dimension less than ''k'' is the face of some simplex \sigma \in \mathcal{K} 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.

A facet is any simplex in a complex that is ''not'' the face of any larger simplex. (Note the difference from the "facet" of a simplex.) A pure simplicial complex can be thought of as a complex where all facets have the same dimension.

Sometimes the term ''face'' is used to refer to a simplex of a complex, not to be confused with the face of a simplex.

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 Homeomorphic to a simplex, leading to the definition of Cell Complex .

The underlying Space , sometimes called the '''carrier''' of a simplicial complex is the Union of its simplices.


CLOSURE, STAR, AND LINK




The closure of a set of simplices ''S'' (denoted Cl ''S'') is the smallest simplicial complex containing all the simplices in S. In other words, Cl ''S'' is the set containing all faces of every simplex in ''S''.

The star of a set of simplices ''S'' (denoted St ''S'') with respect to a simplicial complex ''K'' is the set of all simplices in ''K'' which have simplices in ''S'' as faces. (Note that the star is not necessarily a simplicial complex.)

The link of a set of simplices ''S'' (denoted Lk ''S'') with respect to a simplicial complex ''K'' equals Cl St ''S'' - St Cl ''S''. The link of ''S'' is in a sense the "boundary" of S with respect to K.


ALGEBRAIC TOPOLOGY


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 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.


COMBINATORICS


Combinatorists often study the f-vector of a simplicial d-complex \Delta, which is the Integral sequence (f_0, f_1, f_2, ..., f_{d+1}), where fi is the number of (i-1)-dimensional faces of \Delta (by convention, f0=1 unless \Delta is the empty complex). For instance, if \Delta is the boundary of the Octahedron , then its f-vector is (1, 6, 12, 8), and if \Delta 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 .

By using the f-vector of a simplicial d-complex \Delta as coefficients of a Polynomial (written in decreasing order of exponents), we obtain the f-polynomial of \Delta. In our two examples above, the f-polynomials would be x^3+6x^2+12x+8 and x^4+18x^3+23x^2+8x+1, respectively.

Combinatorists are often quite interested in the h-vector of a simplicial complex \Delta, which is the sequence of coefficients of the polynomial that results from plugging x-1 into the f-polynomial of \Delta. Formally, if we write F_\Delta (x) to mean the f-polynomial of \Delta, then the '''h-polynomial''' of \Delta is

:F_\Delta(x-1)=h_0x^{d+1}+h_1x^d+h_2x^{d-1}+...+h_dx+h_{d+1}

and the h-vector of \Delta is (h_0, h_1, h_2, ..., h_{d+1}). We calculate the h-vector of the octahedron boundary (our first example) as follows:

:F(x-1)=(x-1)^3+6(x-1)^2+12(x-1)+8=x^3+3x^2+3x+1

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 \Delta 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 \Delta to be the 2-complex given by two triangles intersecting only at a common vertex, the resulting h-vector is (1,3,-2).

A complete characterization of all simplicial polytope h-vectors is given by the celebrated G-theorem of Stanley , Billera, and Lee.


SEE ALSO