2011 AMC 10A Problems/Problem 22
Each vertex of convex pentagon is to be assigned a color. There are colors to choose from, and the ends of each diagonal must have different colors. How many different colorings are possible?
Let vertex be any vertex, then vertex be one of the diagonal vertices to , be one of the diagonal vertices to , and so on. We consider cases for this problem.
In the case that has the same color as , has a different color from and so has a different color from and . In this case, has choices, has choices (any color but the color of ), has choice, has choices, and has choices, resulting in a possible of combinations.
In the case that has a different color from and has a different color from , has choices, has choices, has choices (since and necessarily have different colors), has choices, and has choices, resulting in a possible of combinations.
In the case that has a different color from and has the same color as , has choices, has choices, has choices, has choice, and has choices, resulting in a possible of combinations.
Adding all those combinations up, we get .
First, notice that there can be cases. One with all vertices painted different colors, one with one pair of adjacent vertices painted the same color and the final one with two pairs of adjacent vertices painted two different colors.
Case : There are ways of assigning each vertex a different color.
Case : There are ways. After picking four colors, we can rotate our pentagon in ways to get different outcomes.
Case : There are ways of arranging the final case. We can pick colors for our pentagon. There are spots for the first pair of colors. Then, there are possible ways we can put the final pair in the last spaces. But because the two pairs are indistinguishable, we divide by .
Adding all the possibilities up, we get
There are ways to assign a color to . WLOG, give vertex a color; we can multiply by at the end. Since vertices and cannot have the same color, there are ways to assign colors to vertex . Using this same logic, there are ways to assign a color to vertices , , and , giving a total of ways. However, vertex cannot be the same color as vertex . To use complementary counting, we need to find the amount of ways for and to have the same color. Notice that this is equivalent to the amount of ways to arrange colors among the vertices of a square. Using the same logic as above, there are ways, except we must subtract the number of ways for a triangle. Each time, there is less vertex, so times less ways to color. This process stops when there are only vertices left; in this case there are simply ways to color this figure. So in conclusion, there are ways.
This problem is a direct application of the chromatic polynomial of a graph, represented by , which returns a polynomial representing the number of ways to color a graph with colors such that no two adjacent vertices share the same color. In other words, it gives us a polynomial in terms of where we can plug in to get our answer.
Well, if we want to find for a graph , we should probably first draw the graph, right? We can first draw five vertices (without any edges) in the shape of a pentagon. After connecting the diagonals, we get a star. We attempt to shift the vertices around to simplify the graph, which we quickly realize is isomorphic (can be turned into) the five-cycle, or the pentagon.
Finding on a graph with a cycle of length at most 3 is straightforward -- we first pick a vertex v. It has 0 colored (visited) neighbors, so we can color it in k ways. We then move on to the vertices adjacent to v, etc, and at the end we multiply all these together. For example, the chromatic polynomial of a triangle is . However, cycles with length > 3 introduce ambiguity, and thus the above technique fails. Thus, we need to use the recursive formula , where represents removing e and represents contracting e, or collapsing the two endpoints of e into one. When we hit a graph where the longest cycle has length 3, we can use the first technique to quickly find .
After about four iterations of the algorithm, we get that the chromatic polynomial is . Plugging in 6 gives us .
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