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prove that $G_n$ contains $C_4$

Source: 12-th Hungary-Israel Mathematical Competition 2001

April 16, 2007
graph theorycombinatorics proposedcombinatorics

Problem Statement

Here GnG_{n} denotes a simple undirected graph with nn vertices, KnK_{n} denotes the complete graph with nn vertices, Kn,mK_{n,m} the complete bipartite graph whose components have mm and nn vertices, and CnC_{n} a circuit with nn vertices. The number of edges in the graph GnG_{n} is denoted e(Gn)e(G_{n}). If e(Gn)nn2+n4e(G_{n}) \geq\frac{n\sqrt{n}}{2}+\frac{n}{4} ,prove that GnG_{n} contains C4C_{4} .