A complete graph K 2 0 1 4 with 2 0 1 4 vertices is drawn on the plane. A series of straight lines is then drawn on the plane, such that if a line crosses through an edge of the graph, then that edge is considered "cut". What is the minimum number of straight lines needed to cut all the edges of the graph K 2 0 1 4 ?
Details and Assumptions
No line crosses through a vertex.
No three vertices are collinear.
The vertices of K 2 0 1 4 need not to be spaced evenly apart; they can be anywhere on the plane. In other words, you are allowed to try to optimize the number of lines needed by picking where the vertices should be.
As an example, 4 lines are necessary to cut all of K 8 's edges. One way to cut it is shown below:
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It was a bit hard for me understand this problem (like what it was asking). :(
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What part was hard to understand? I can fix it.
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That illustration of yours made it much better. :)
Your problem statement is missing an important detail.
Typically, in a combinatorics problem like this, you would have to give the minimum number of cuts, no matter where the 2014 points are located. (In other words, consider an adversary who wants to maximize the number of cuts you need to make. In this case, I believe the answer is 1007.)
However, in your solution, you get to choose where to place the 2014 points, which makes it a very different problem. You need to make this clear in the problem statement.
Why couldnt I put first place a square perimeter using 4 vertices, then I include all the other vertices within the perimeter, such that there are only 4 EDGES, hence only needing 2 lines to cut thru all the edges.
Please advice?
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Yes, at least change the word "edge" in the question. This is a trap!
I addressed that in the Details and assumptions now.
However, I still don't see how that would only take 2 lines to cut through it...
You found a necessary bound, but is it sufficient?
I believe that the answer is 1006.
Hint: Draw the circle that these 2014 points lie on. Consider how these lines cut up the circle.
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I'm pretty sure that it is sufficient. Simply divide the plane using the 6 3 lines into 2 0 1 7 regions (the maximum possible), and place the 2 0 1 4 vertices, one in each of the regions. Since each of these vertices is in its own section, no two vertices can have their common edge still intact; there will always be at least one line between them.
Note in the Details and Assumptions:
The vertices of K 2 0 1 4 need not to be spaced evenly apart; they can be anywhere on the plane.
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Ah, I missed that condition. I was thinking that it had to be the graph in the image. Thanks!
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Lemma: there must be a line between any two vertices.
Proof: suppose this is not true, that there are two vertices without a line between them. Then their shared edge would not be cut, contradiction.
Lemma: the number of sections the lines divide the plane must be equal or more than the number of vertices, 2 0 1 4 .
Proof: suppose that we have less sections of the plane than vertices. Then, by Pigeonhole Principle, we must have at least 2 vertices in a section of the graph. But then these two vertices, being part of the same section, cannot have a line between them, so their edge is not cut, contradiction.
Thus, for n straight lines, if the maximum number of sections they divide the plane is less than 2 0 1 4 , then it is not possible to cut all edges with n lines.
Now we just have to calculate the smallest n such that the number of sections they create is more than 2 0 1 4 .
First off, the formula for the maximum number of regions cut with n lines is 2 n 2 + n + 2 .
Thus, we want 2 n 2 + n + 2 ≥ 2 0 1 4
n 2 + n + 2 ≥ 4 0 2 8
n 2 + n ≥ 4 0 2 6
We approximate the square root of 4 0 2 6 to be ≈ 6 3 → 6 4 , so we check n around this region.
Checking for n = 6 2 , we see that 6 2 2 + 6 2 = 3 0 9 6 < 4 0 2 8
Checking for n = 6 3 , we see that 6 3 2 + 6 3 = 4 0 3 2 > 4 0 2 8 , so the correct answer is n = 6 3 .
Note that we barely made it n = 6 3 ; if we, instead, had 2 0 1 8 vertices, then n = 6 4 is necessary.