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Combinatorics Difficulty 7.5 National olympiad, round 2 Prove it Turkey

Alice, who works for the Graph County Electric Works, is commissioned to wire the newly erected utility poles in kk days. Each day she either chooses a pole and runs wires from it to as many poles as she wishes, or chooses at most 1717 pairs of poles and runs wires between each pair. Bob, who works for the Graph County Paint Works, claims that, no matter how many poles there are and how Alice connects them, all the poles can be painted using not more than 20092009 colors in such a way that no pair of poles connected by a wire is the same color. Determine the greatest value of kk for which Bob's claim is valid.

Solution

Let G=i=1kGiG = \bigcup_{i=1}^{k} G_i be a graph that can be vertex-colored using not more than 20092009 colors where each GiG_i is either a star or e(Gi)17e(G_i) \le 17 for 1ik1 \le i \le k. We want to find the maximum possible value of kk.

Suppose that GiG_i is a star for 1ij1 \le i \le j, and GiG_i satisfies e(Gi)17e(G_i) \le 17 for j+1ikj+1 \le i \le k where 0jk0 \le j \le k. If at least j+10j+10 colors are needed to color i=1jGi\bigcup_{i=1}^{j} G_i, then we must have (j+102)17j\binom{j+10}{2} \le 17j and since (j152)2+13540(j - \frac{15}{2})^2 + \frac{135}{4} \le 0, this gives a contradiction. Hence it is possible to color i=1jGi\bigcup_{i=1}^{j} G_i using at most j+9j+9 colors. Since we might

need at most one more color every time we add a star, we conclude that GG can
be colored using not more than k+9k + 9 colors.

On the other hand, if k9k \ge 9, there is a case when k+9k+9 is needed: We use 99 GiG_is to form a C18C_{18} and then use k9k-9 stars to complete this to a Ck+9C_{k+9}.

We conclude that greatest value of kk for which 20092009 colors suffice is 20002000.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.