Olympiad Maths Prep

Library / /45 of 45

Combinatorics Difficulty 7.9 National olympiad, round 2 Prove it Ukraine

In a country there are 20242024 cities, each two of which are connected bidirectionally by exactly one of three modes of transportation: rail, air, or road. A tourist arrives in this country and has the entire transportation scheme. He chooses a travel ticket for one of the modes of transportation and the city from which he starts his trip. He wants to visit as many cities as possible but using only the ticket for the specified type of transportation. How many cities is a tourist guaranteed to be able to visit? During the route, he can return to the cities he has already visited.

(Bogdan Rublov)

Figure 1
Fig. 20

Solution

Suppose we have a complete graph with each edge colored in one of three colors. We show that there will always exist a connected component of at least one of the colors of size at least 10121012. First, we show that it is possible to color the edges in such a way that connected components of size more than 10121012 vertices do not exist. Let's label the colors with numbers - 1st, 2nd and 3rd. Let's divide the vertices into groups by 506506 - groups "A", "B", "C" and "D". Let all 10121012 vertices of groups "A" and "B" be connected by edges of color 1, all vertices of group "C" - by color 2, group "D" - by color 3. Between them, all vertices are connected by edges of those colors, as shown in fig. 20. Therefore, groups "A" or "B" form a connected component of size 10121012 of color 1. Groups "A" and "C" are of color 2, groups "B" and "D" are of color 3. Obviously, there is no larger connected component in terms of the number of vertices.

Suppose that there exists a coloring such that the largest connected component contains fewer than 10121012 vertices. Let's consider the coloring that gives the smallest answer to the problem.

Consider the largest connected component of one of the colors. This component, or rather the number of vertices in it, is the answer to the problem. If there are several largest components, choose any of them. Let this component correspond to color 1. Let's denote this component as I. Let this component contain exactly mm vertices. Note that not all the edges between the vertices of this component have the color 1. But if we repaint all the edges of this component with color 1, then the answer mm in the problem will not change. In this case, there are no vertices of the component I from which an edge of color 1 can come out outside this component I, that is, to a vertex that does not belong to the component I.

Next, consider the largest connected component among all other vertices, II, with color 2 or 3. It includes all the vertices that are connected by color 2, i.e., also those that can be reached through the vertices of the component I. For example, if AA \in II, BB \in I, and for the vertex CC \notin I the following conditions are true: A(II) 2B(I) 2CA(II)\ 2 \leftrightarrow B(I)\ 2 \leftrightarrow C, then we assume that CC \in II, but we do not assume that BB \in II. In other words, a vertex cannot belong to two components at the same time (this principle will be maintained in the future). The number of vertices in the component II is denoted by kk. Now we can repaint all edges in this component into color 2, and, again, the answer mm in the problem will not change.

Now let's denote by X2X_2 the group of vertices of the component I that are connected to at least one vertex of the component II by an edge of color 2, let their number be x2x_2. Then all the edges between the vertices X2X_2 and the vertices of component II can be colored in color 2. They still form the connected component of color 2 before this coloring, but since the component of color 1 was chosen first, the total number of vertices k+x2mk + x_2 \le m.

Then all the vertices of the component I that are not included in the group X2X_2, we will combine into the group X3X_3 and denote their number by x3x_3. From these vertices, only edges of color 3 can go to each vertex of component II. But then the vertices of the group X3X_3 and the component II form a connected component of color 3. But then from the vertices of the group X3X_3 there cannot be any edges of color 1 or 3 to vertices that are not included in the components I and II. If there is an edge of color 3, then the connected component of color 3 will be larger than the component II, and this contradicts its construction. Therefore, all vertices that are not included in the components I and II are connected by color 2 (since X3X_3 is not empty). We denote all these vertices as a component III. Similarly, the vertices of the group X2X_2 are connected with III only with edges of color 3. But then the components II and III should be connected only by edges of color 1. That is, they form a connected component of color 1. But we initially chose the largest connected component of color 1, which has mm vertices. Therefore, groups II and III groups have no more than mm vertices, hence 2m20242m \ge 2024 and this completes the proof.

Looking for a route rather than an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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