Introduction Ramsey numbers for graph classes Ramsey numbers for line graphs and perfect graphs Pim van 't Hof University of Bergen joint work with R´emyBelmonte University of Bergen Pinar Heggernes University of Bergen Reza Saei University of Bergen COCOON 2012 Sydney, Australia August 20{22, 2012 R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs August 21, 2012 Observation R(i; j) = R(j; i) for every pair of positive integers (i; j). Introduction Ramsey numbers for graph classes Ramsey numbers Definition For any pair of positive integers (i; j), the Ramsey number R(i; j) is the smallest integer p such that every graph on p vertices contains a clique of size i or an independent set of size j. Theorem (Ramsey; 1930) For every pair of positive integers (i; j), the number R(i; j) exists. R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Ramsey numbers for graph classes Ramsey numbers Definition For any pair of positive integers (i; j), the Ramsey number R(i; j) is the smallest integer p such that every graph on p vertices contains a clique of size i or an independent set of size j. Theorem (Ramsey; 1930) For every pair of positive integers (i; j), the number R(i; j) exists. Observation R(i; j) = R(j; i) for every pair of positive integers (i; j). R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs j 3 4 5 6 7 8 9 10 i 3 6 9 14 18 23 28 36 40-43 4 9 18 25 35-41 49-61 56-84 73-115 92-149 5 14 25 43-49 58-87 80-143 101-216 125-316 143-442 6 18 35-41 58-87 102-165 113-298 127-495 169-780 179-1171 Introduction Ramsey numbers for graph classes Known (bounds on) Ramsey numbers Observation R(1; j) = 1 and R(2; j) = j for every integer j ≥ 1. R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Ramsey numbers for graph classes Known (bounds on) Ramsey numbers Observation R(1; j) = 1 and R(2; j) = j for every integer j ≥ 1. j 3 4 5 6 7 8 9 10 i 3 6 9 14 18 23 28 36 40-43 4 9 18 25 35-41 49-61 56-84 73-115 92-149 5 14 25 43-49 58-87 80-143 101-216 125-316 143-442 6 18 35-41 58-87 102-165 113-298 127-495 169-780 179-1171 R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Ramsey numbers for graph classes Known (bounds on) Ramsey numbers Observation R(1; j) = 1 and R(2; j) = j for every integer j ≥ 1. j 3 4 5 6 7 8 9 10 i 3 6 9 14 18 23 28 36 40-43 4 9 18 25 35-41 49-61 56-84 73-115 92-149 5 14 25 43-49 58-87 80-143 101-216 125-316 143-442 6 18 35-41 58-87 102-165 113-298 127-495 169-780 179-1171 R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Ramsey numbers for graph classes Known (bounds on) Ramsey numbers Observation R(1; j) = 1 and R(2; j) = j for every integer j ≥ 1. j 3 4 5 6 7 8 9 10 i 3 6 9 14 18 23 28 36 40-43 4 9 18 25 35-41 49-61 56-84 73-115 92-149 5 14 25 43-49 58-87 80-143 101-216 125-316 143-442 6 18 35-41 58-87 102-165 113-298 127-495 169-780 179-1171 \Imagine an alien force, vastly more power- ful than us, landing on Earth and demand- ing the value of R(5; 5) or they will destroy our planet. In that case, we should marshal all our computers and all our mathematicians and attempt to find the value." R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Ramsey numbers for graph classes Known (bounds on) Ramsey numbers Observation R(1; j) = 1 and R(2; j) = j for every integer j ≥ 1. j 3 4 5 6 7 8 9 10 i 3 6 9 14 18 23 28 36 40-43 4 9 18 25 35-41 49-61 56-84 73-115 92-149 5 14 25 43-49 58-87 80-143 101-216 125-316 143-442 6 18 35-41 58-87 102-165 113-298 127-495 169-780 179-1171 \But suppose, instead, that they ask for R(6; 6). In that case, we should attempt to destroy the aliens." R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Goal Identify graph classes G for which the value RG(i; j) can be determined for every pair of positive integers (i; j). Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Ramsey numbers for graph classes Definition Let G be a class of graphs. For any pair of positive integers (i; j), the Ramsey number RG(i; j) is the smallest integer p such that every graph on p vertices that belongs to G contains a clique of size i or an independent set of size j. Observation For any graph class G and every pair of positive integers (i; j), RG(i; j) ≤ R(i; j). R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Ramsey numbers for graph classes Definition Let G be a class of graphs. For any pair of positive integers (i; j), the Ramsey number RG(i; j) is the smallest integer p such that every graph on p vertices that belongs to G contains a clique of size i or an independent set of size j. Observation For any graph class G and every pair of positive integers (i; j), RG(i; j) ≤ R(i; j). Goal Identify graph classes G for which the value RG(i; j) can be determined for every pair of positive integers (i; j). R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Theorem (Steinberg & Tovey; JCTB 1993) The planar Ramsey numbers are: RP (2; j) = j for j ≥ 1 and RP (i; 2) = i for i ≤ 5. RP (3; j) = 3j − 3 for j ≥ 2. RP (i; j) = 4j − 3 for i ≥ 4 and (i; j) 6= (4; 2). Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Known results on Ramsey numbers for graph classes Theorem (Walker; 1969) Let P be the class of planar graphs. Then RP (2; j) = j for j ≥ 1 and RP (i; 2) = i for i ≤ 5; RP (3; j) = 3j − 3 for j ≥ 2; 4j − 3 ≤ RP (i; j) ≤ 5j − 4 for i ≥ 4 and (i; j) 6= (4; 2). Moreover, the truth of the four-color conjecture would imply that RP (i; j) = 4j − 3 for i ≥ 4 and (i; j) 6= (4; 2). R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Known results on Ramsey numbers for graph classes Theorem (Walker; 1969) Let P be the class of planar graphs. Then RP (2; j) = j for j ≥ 1 and RP (i; 2) = i for i ≤ 5; RP (3; j) = 3j − 3 for j ≥ 2; 4j − 3 ≤ RP (i; j) ≤ 5j − 4 for i ≥ 4 and (i; j) 6= (4; 2). Moreover, the truth of the four-color conjecture would imply that RP (i; j) = 4j − 3 for i ≥ 4 and (i; j) 6= (4; 2). Theorem (Steinberg & Tovey; JCTB 1993) The planar Ramsey numbers are: RP (2; j) = j for j ≥ 1 and RP (i; 2) = i for i ≤ 5. RP (3; j) = 3j − 3 for j ≥ 2. RP (i; j) = 4j − 3 for i ≥ 4 and (i; j) 6= (4; 2). R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Known results on Ramsey numbers for graph classes Let C be the class of claw-free graphs. Theorem (Matthews; 1985) RC(i; 3) = R(i; 3) for every positive integer i. Proof. R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Known results on Ramsey numbers for graph classes Let C be the class of claw-free graphs. Theorem (Matthews; 1985) RC(i; 3) = R(i; 3) for every positive integer i. Proof. RC(i; 3) ≤ R(i; 3) by definition. R. Belmonte, P. Heggernes, P. van 't Hof, R. Saei Ramsey numbers for line graphs and perfect graphs Let G be a graph on R(i; 3) − 1 vertices that has no Ki and no K3. B G is claw-free. Introduction Perfect graphs Ramsey numbers for graph classes Line graphs Known results on Ramsey numbers for graph classes Let C be the class of claw-free graphs.