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Combinatorics Difficulty 8.9 Shortlist Find the answer

Find the largest positive integer kk{} for which there exists a convex polyhedron P\mathcal{P} with 2022 edges, which satisfies the following properties:
[list]
[*]The degrees of the vertices of P\mathcal{P} don’t differ by more than one, and
[*]It is possible to colour the edges of P\mathcal{P} with kk{} colours such that for every colour cc{}, and every pair of vertices (v1,v2)(v_1, v_2) of P\mathcal{P}, there is a monochromatic path between v1v_1 and v2v_2 in the colour cc{}.
[/list]
[i]Viktor Simjanoski, Macedonia[/i]

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Solution

We are tasked with finding the largest positive integer k k such that there exists a convex polyhedron P\mathcal{P} with 2022 edges, which satisfies the following conditions:
1. The degrees of the vertices of P\mathcal{P} do not differ by more than one.
2. It is possible to color the edges of P\mathcal{P} with k k colors such that for every color c c and every pair of vertices (v1,v2)(v_1, v_2) of P\mathcal{P}, there is a monochromatic path between v1 v_1 and v2 v_2 in the color c c .

### Step-by-step Solution:

1. Euler's Formula: For a convex polyhedron, Euler's formula states:
VE+F=2 V - E + F = 2
where V V is the number of vertices, E E is the number of edges, and F F is the number of faces. Given E=2022 E = 2022 , we apply this formula.

2. Vertex Degree Property: If the vertex degrees do not differ by more than one, and given that the sum of the degrees of all vertices equals twice the number of edges (since each edge is incident to two vertices), we have:
i=1Vdeg(vi)=2E=4044 \sum_{i=1}^{V} \deg(v_i) = 2E = 4044
Let the degrees of the vertices be d d and d+1 d+1 . If x x vertices have degree d d and y y vertices have degree d+1 d+1 , then:
xd+y(d+1)=4044 xd + y(d+1) = 4044
x+y=V x + y = V

3. **Solving for d d **: Substitute y=Vx y = V - x into the degree equation:
xd+(Vx)(d+1)=4044 xd + (V - x)(d + 1) = 4044
xd+Vd+Vxdx=4044 xd + Vd + V - xd - x = 4044
Vd+Vx=4044 Vd + V - x = 4044
x=V(4044Vd) x = V - (4044 - Vd)

4. Edge Coloring and Monochromatic Paths: We need a coloring such that there is a monochromatic path for any pair of vertices. Each component in the monochromatic graph should be a tree spanning all vertices. Given that the graph is connected, a valid coloring with k=2 k = 2 is sufficient since every component must span the graph, thus forming two tree structures if k=2 k = 2 .

5. Verification: If k=2 k = 2 , color the edges such that each color spans a tree. This satisfies both the paths and coloring condition. Larger values for k k would complicate forming monochromatic spanning trees since there might not exist distinct spanning subgraphs allowing for more colors.

Hence, the largest value of k k is:
2 \boxed{2}

This solution stems from ensuring the polyhedron's edge conditions and utilizing graph coloring properties to achieve required monochromatic connectivity.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.