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Graph with 7n/4 Edges

Source: Iran 3rd round 2013 - Combinatorics exam problem 5

September 18, 2014
functioncombinatoricsgraph theory

Problem Statement

Consider a graph with nn vertices and 7n4\frac{7n}{4} edges. (a) Prove that there are two cycles of equal length. (25 points) (b) Can you give a smaller function than 7n4\frac{7n}{4} that still fits in part (a)? Prove your claim. We say function a(n)a(n) is smaller than b(n)b(n) if there exists an NN such that for each n>Nn>N ,a(n)<b(n)a(n)<b(n) (At most 5 points) Proposed by Afrooz Jabal'ameli