MATH0029 Graph Theory and Combinatorics
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MATH0029 Graph Theory and Combinatorics
Section A
1. Let X = {1, 2, 3, 4, 5}. Carefully justifying your answers:
(a) Give an example of a chain of maximum size in 伫(X).
(b) Give an example of an antichain of maximum size in 伫(X).
(c) What is the maximum size of a family in 伫(X) that is both a chain and an antichain?
(d) Give a partition of 伫(X) into symmetric chains.
2. (a) State Ramsey’s theorem.
(b) Given that R(3, 4) = 9 prove directly (without using Ramsey’s theorem) that R(3, 5) ≤ 14.
(c) For t > 2 define R4 (t) to be the smallest integer n such that any edge colouring of Kn with 4 colours contains a monochromatic copy of Kt . Prove that for t > 6 we have R4 (t) > 2t .
3. (a) Give an example of a complete bipartite graph of order 6 containing a Hamilton cycle. Justify your answer.
(b) Give an example of a complete bipartite graph of order 6 that does not contain a Hamilton cycle. Justify your answer.
(c) Let n > 2 and define Gn to be the graph with vertex set {一1, +1}n and edges given by
E(Gn ) = {uv | u, v ∈ {一1, +1}n differ in exactly one coordinate}.
(i) For each n > 2 compute χ(Gn ). Justify your answer.
(ii) For which n > 2 does Gn contain an Euler circuit? Justify your answer. (iii) For which n > 2 does Gn contains a Hamilton cycle? Justify your answer.
Section B
4. (a) Let G be a graph of order n > 3. Prove that either G is complete r-partite, for some r > 1, or G contains an induced subgraph isomorphic to H = ({a, b, c}, {ab}).
(b) Call a graph G maximal F -free if G is F-free but the addition of any new edge to G creates a copy of F .
(i) Prove that if G is maximal K3-free then either G is complete bipartite or G contains a copy of C5 .
(ii) Prove that if G is maximal K4-free then either G is complete 3-partite or G contains a copy of either H1 or H2 (see below)
H1 = ({a, s, t, u, v, y, z}, {as, at, au, av, st, uv, yz, sy, ty, uz, vz})
H2 = ({a, s, t, u, y, z}, {as, at, au, st, tu, yz, sy, ty, tz, uz}).
Figure 1: The graphs H1 and H2
5. (a) How many edges does the Tur´an graph T4 (9) contain?
(b) For which values of n > r > 2 does T4 (9) contain a copy of Tr (n)? Justify your answer.
(c) Calculate, with justification, the Tur´an density of each of the following graphs: K3 , K3,3 , T4 (2020).
(d) Let H be a graph with at least one edge and Tur´an density π(H) = α. Let H* be formed from H by adding two new vertices a, b and adding new edges {ab} U {va, vb | v ∈ V (H)}. What is the Tur´an density of H* ? Justify your answer.
2022-05-03