Independence complex

The independence complex of a graph is a mathematical object describing the independent sets of the graph. Formally, the independence complex of an undirected graph G, denoted by I(G), is an abstract simplicial complex (that is, a family of finite sets closed under the operation of taking subsets), formed by the sets of vertices in the independent sets of G. Any subset of an independent set is itself an independent set, so I(G) indeed meets the requirement of an abstract simplicial complex that every subset of a set in the family should also be in the family.

Every independent set in a graph is a clique in its complement graph, and vice versa. Therefore, the independence complex of a graph equals the clique complex of its complement graph, and vice versa.

Homology groups

Several authors studied the relations between the properties of a graph G = (V, E), and the homology groups of its independence complex I(G).[1] In particular, several properties related to the dominating sets in G guarantee that some reduced homology groups of I(G) are trivial.

1. The total domination number of G, denoted ${\displaystyle \gamma _{0}(G)}$, is the minimum cardinality of a set total dominating set of G - a set S such that every vertex of V is adjacent to a vertex of S. If ${\displaystyle \gamma _{0}(G)>k}$ then ${\displaystyle {\tilde {H}}_{k-1}(I(G))=0}$.[2]

2. The total domination number of a subset A of V in G, denoted ${\displaystyle \gamma _{0}(G,A)}$, is the minimum cardinality of a set S such that every vertex of A is adjacent to a vertex of S. The independence domination number of G, denoted ${\displaystyle i\gamma (G)}$, is the maximum, over all independent sets A in G, of ${\displaystyle \gamma _{0}(G,A)}$. If ${\displaystyle i\gamma (G)>k}$, then ${\displaystyle {\tilde {H}}_{k-1}(I(G))=0}$.[1][3]

3. The domination number of G, denoted ${\displaystyle \gamma (G)}$, is the minimum cardinality of a dominating set of G - a set S such that every vertex of V \ S is adjacent to a vertex of S. Note that ${\displaystyle \gamma _{0}(G)\geq \gamma (G)}$. If G is a chordal graph and ${\displaystyle \gamma (G)>k}$ then ${\displaystyle {\tilde {H}}_{k-1}(I(G))=0}$.[4]

4. The induced matching number of G, denoted ${\displaystyle \mu (G)}$, is the largest cardinality of an induced matching in G - a matching that includes every edge connecting any two vertices in the subset. If there exists a subset A of V such that ${\displaystyle \gamma _{0}(G,A)>k+\min[k,\mu (G[A])]}$ then ${\displaystyle {\tilde {H}}_{k-1}(I(G))=0}$.[5] This is a generalization of both properties 1 and 2 above.

5. The non-dominating independence complex of G, denoted I'(G), is the abstract simplicial complex of the independent sets that are not dominating sets of G. Obviously I'(G) is contained in I(G); denote the inclusion map by ${\displaystyle i:I'(G)\to I(G)}$. If G is a chordal graph then the induced map ${\displaystyle i_{*}:{\tilde {H}}_{k}(I'(G))\to {\tilde {H}}_{k}(I(G))}$ is zero for all ${\displaystyle k\geq -1}$.[1]^:Thm.1.4 This is a generalization of property 3 above.

6. The fractional star-domination number of G, denoted ${\displaystyle \gamma _{s}^{*}(G)}$, is the minimum size of a fractional star-dominating set in G. If ${\displaystyle \gamma _{s}^{*}(G)>k}$ then ${\displaystyle {\tilde {H}}_{k-1}(I(G))=0}$.[1]^:Thm.1.5

Related concepts

Meshulam's game is a game played on a graph G, that can be used to calculate a lower bound on the homological connectivity of the independence complex of G.

The matching complex of a graph G, denoted M(G), is an abstract simplicial complex of the matchings in G. It is the independence complex of the line graph of G.[6][7]

The (m,n)-chessboard complex is the matching complex on the complete bipartite graph K_m,_n. It is the abstract simplicial complex of all sets of positions on an m-by-n chessboard, on which it is possible to put rooks without each of them threatening the other.[8][9]

The clique complex of G is the independence complex of the complement graph of G.

See also

• Rainbow-independent set

References

1. Meshulam, Roy (2003-05-01). "Domination numbers and homology". Journal of Combinatorial Theory, Series A. 102 (2): 321–330. doi:10.1016/S0097-3165(03)00045-1. ISSN 0097-3165.
2. Chudnovsky, Maria (2000). Systems of disjoint representatives (M.Sc. thesis). Haifa, Israel: Technion, department of mathematics.
3. Aharoni, Ron; Haxell, Penny (2000). "Hall's theorem for hypergraphs". Journal of Graph Theory. 35 (2): 83–88. doi:10.1002/1097-0118(200010)35:2<83::aid-jgt2>3.0.co;2-v. ISSN 0364-9024.
4. Aharoni, Ron; Berger, Eli; Ziv, Ran (2002-07-01). "A Tree Version of Kőnig's Theorem". Combinatorica. 22 (3): 335–343. doi:10.1007/s004930200016. ISSN 0209-9683. S2CID 38277360.
5. Meshulam, Roy (2001-01-01). "The Clique Complex and Hypergraph Matching". Combinatorica. 21 (1): 89–94. doi:10.1007/s004930170006. ISSN 1439-6912. S2CID 207006642.
6. Björner, A.; Lovász, L.; Vrećica, S. T.; Živaljević, R. T. (1994). "Chessboard Complexes and Matching Complexes". Journal of the London Mathematical Society. 49 (1): 25–39. doi:10.1112/jlms/49.1.25. ISSN 1469-7750.
7. Reiner, Victor; Roberts, Joel (2000-03-01). "Minimal Resolutions and the Homology of Matching and Chessboard Complexes". Journal of Algebraic Combinatorics. 11 (2): 135–154. doi:10.1023/A:1008728115910. ISSN 1572-9192.
8. Friedman, Joel; Hanlon, Phil (1998-09-01). "On the Betti Numbers of Chessboard Complexes". Journal of Algebraic Combinatorics. 8 (2): 193–203. doi:10.1023/A:1008693929682. ISSN 1572-9192.
9. Ziegler, Günter M. (1994-02-01). "Shellability of chessboard complexes". Israel Journal of Mathematics. 87 (1): 97–110. doi:10.1007/BF02772986. ISSN 1565-8511. S2CID 59040033.