Characterization by forbidden induced graphs of some subclasses of chordal graphs S´ergioH. Nogueira1;2 e Vinicius F. dos Santos1;3 1 PPGMMC, CEFET-MG 2 Instituto de Ci^enciasExatas e Tecnol´ogicas,UFV 3 Departamento de Ci^enciada Computa¸c~ao,UFMG La Plata, Argentina November, 2016 Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 1 / 15 Definition A graph is chordal if every cycle of length greater than three has a chord*. *An edge connecting two nonconsecutive vertices. Minimal separators Definition A set S ⊂ V (G) disconnects a vertex a from b in G if every path of G between a and b contains a vertex from S. A non-empty set S ⊂ V (G) is a minimal separator of G if there exist a and b such that S disconnects a from b in G and no proper subset of S disconnects a from b in G. Definitions Definitions Chordal graph Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 2 / 15 Minimal separators Definition A set S ⊂ V (G) disconnects a vertex a from b in G if every path of G between a and b contains a vertex from S. A non-empty set S ⊂ V (G) is a minimal separator of G if there exist a and b such that S disconnects a from b in G and no proper subset of S disconnects a from b in G. Definitions Definitions Chordal graph Definition A graph is chordal if every cycle of length greater than three has a chord*. *An edge connecting two nonconsecutive vertices. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 2 / 15 Definition A set S ⊂ V (G) disconnects a vertex a from b in G if every path of G between a and b contains a vertex from S. A non-empty set S ⊂ V (G) is a minimal separator of G if there exist a and b such that S disconnects a from b in G and no proper subset of S disconnects a from b in G. Definitions Definitions Chordal graph Definition A graph is chordal if every cycle of length greater than three has a chord*. *An edge connecting two nonconsecutive vertices. Minimal separators Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 2 / 15 Definitions Definitions Chordal graph Definition A graph is chordal if every cycle of length greater than three has a chord*. *An edge connecting two nonconsecutive vertices. Minimal separators Definition A set S ⊂ V (G) disconnects a vertex a from b in G if every path of G between a and b contains a vertex from S. A non-empty set S ⊂ V (G) is a minimal separator of G if there exist a and b such that S disconnects a from b in G and no proper subset of S disconnects a from b in G. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 2 / 15 Definition A clique tree of a connected chordal graph is any tree T whose vertices are the maximal cliques of G such that for every two cliques C1; C2 each clique on the path from C1 to C2 in T contains C1 \ C2. Separating pair Definition Two maximal cliques C1; C2 of G form a separating pair if C1 \ C2 is non-empty, and every path in G from a vertex of C1nC2 to a vertex of C2nC1 contains a vertex of C1 \ C2. Definitions Clique tree Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 3 / 15 Separating pair Definition Two maximal cliques C1; C2 of G form a separating pair if C1 \ C2 is non-empty, and every path in G from a vertex of C1nC2 to a vertex of C2nC1 contains a vertex of C1 \ C2. Definitions Clique tree Definition A clique tree of a connected chordal graph is any tree T whose vertices are the maximal cliques of G such that for every two cliques C1; C2 each clique on the path from C1 to C2 in T contains C1 \ C2. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 3 / 15 Definition Two maximal cliques C1; C2 of G form a separating pair if C1 \ C2 is non-empty, and every path in G from a vertex of C1nC2 to a vertex of C2nC1 contains a vertex of C1 \ C2. Definitions Clique tree Definition A clique tree of a connected chordal graph is any tree T whose vertices are the maximal cliques of G such that for every two cliques C1; C2 each clique on the path from C1 to C2 in T contains C1 \ C2. Separating pair Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 3 / 15 Definitions Clique tree Definition A clique tree of a connected chordal graph is any tree T whose vertices are the maximal cliques of G such that for every two cliques C1; C2 each clique on the path from C1 to C2 in T contains C1 \ C2. Separating pair Definition Two maximal cliques C1; C2 of G form a separating pair if C1 \ C2 is non-empty, and every path in G from a vertex of C1nC2 to a vertex of C2nC1 contains a vertex of C1 \ C2. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 3 / 15 Definitions Clique tree Definition A clique tree of a connected chordal graph is any tree T whose vertices are the maximal cliques of G such that for every two cliques C1; C2 each clique on the path from C1 to C2 in T contains C1 \ C2. Separating pair Definition Two maximal cliques C1; C2 of G form a separating pair if C1 \ C2 is non-empty, and every path in G from a vertex of C1nC2 to a vertex of C2nC1 contains a vertex of C1 \ C2. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 3 / 15 Previous results Main results Theorem A graph G is chordal if and only if every minimal separator of G is a clique. (Golumbic, 2004) [4] Theorem Let G be a chordal graph. The multiset S of minimal separators of vertices of G is the same for every clique tree T of G.(Blair and Peyton, 1992) [1] Theorem A set S is a minimal separator of a chordal graph G if and only if there exist maximal cliques C1; C2 forming a separating pair such that S = C1 \ C2. (Habib and Stacho, 2012) [5] Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 4 / 15 Goals Goal Our goal is to characterize a subclass of chordal graphs by the intersection of minimal separators and forbidden induced subgraphs. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 5 / 15 . Other results: . (ii) (For every Si ; Sj 2 S; Si \ Sj 6= ;) Si = Sj ) , G is (dart, gem)-free. (strictly chordal graphs) Markezon and Waga, 2015) [6], [7]. (iii) (For every Si ; Sj 2 S; i 6= j; Si \ Sj 6= ;) Si Sj or Sj Si ) , G is gem-free. (iv) Π4: For every Si ; Sj 2 S; i 6= j; Si \ Sj 6= ;) Si * Sj and Sj * Si : A chordal graph is Π4 hereditary , G is dart-free. ((() De Caria and Guti´errez,2016) [3] Results Theorem Theorem Let G be a chordal graph and let S = fS1; S2; ; :::; Sng the multiset of minimal separators of G. Then: (i) For every Si ; Sj 2 S; i 6= j; Si \ Sj = ;, G is (claw, gem)-free. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 6 / 15 Results Theorem Theorem Let G be a chordal graph and let S = fS1; S2; ; :::; Sng the multiset of minimal separators of G. Then: (i) For every Si ; Sj 2 S; i 6= j; Si \ Sj = ;, G is (claw, gem)-free. Other results: . (ii) (For every Si ; Sj 2 S; Si \ Sj 6= ;) Si = Sj ) , G is (dart, gem)-free. (strictly chordal graphs) Markezon and Waga, 2015) [6], [7]. (iii) (For every Si ; Sj 2 S; i 6= j; Si \ Sj 6= ;) Si Sj or Sj Si ) , G is gem-free. (iv) Π4: For every Si ; Sj 2 S; i 6= j; Si \ Sj 6= ;) Si * Sj and Sj * Si : A chordal graph is Π4 hereditary , G is dart-free. ((() De Caria and Guti´errez,2016) [3] Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 6 / 15 Results Proof Si \ Sj = ;, G is (claw, gem)-free. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 7 / 15 Results Proof Si \ Sj = ;, G is (claw, gem)-free. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 7 / 15 Results ()) Suppose that G has a claw or a gem. If it has a claw, let x; y; z; t be the vertices of the claw and let C1; C2; C3 cliques containing fx; yg; fx; zg; fx; tg, respectively. Nogueira e dos Santos (CEFET-MG, UFV, DCC-UFMG)Characterization of subclasses of chordal graphs November, 2016 8 / 15 Results We can have other situations, as But it is analogous.