Extreme Cut-the-Rope

A complete graph K 2014 K_{2014} with 2014 2014 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 2014 K_{2014} ?

Details and Assumptions

No line crosses through a vertex.

No three vertices are collinear.

The vertices of K 2014 K_{2014} 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 4 lines are necessary to cut all of K 8 K_8 's edges. One way to cut it is shown below:


The answer is 63.

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1 solution

Daniel Liu
Jun 15, 2014

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, 2014 2014 .

Proof: suppose that we have less sections of the plane than vertices. Then, by Pigeonhole Principle, we must have at least 2 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 n straight lines, if the maximum number of sections they divide the plane is less than 2014 2014 , then it is not possible to cut all edges with n n lines.

Now we just have to calculate the smallest n n such that the number of sections they create is more than 2014 2014 .

First off, the formula for the maximum number of regions cut with n n lines is n 2 + n + 2 2 \dfrac{n^2+n+2}{2} .

Thus, we want n 2 + n + 2 2 2014 \dfrac{n^2+n+2}{2}\ge 2014

n 2 + n + 2 4028 n^2+n+2\ge 4028

n 2 + n 4026 n^2+n\ge 4026

We approximate the square root of 4026 4026 to be 63 64 \approx 63\to 64 , so we check n n around this region.

Checking for n = 62 n=62 , we see that 6 2 2 + 62 = 3096 < 4028 62^2+62=3096 < 4028

Checking for n = 63 n=63 , we see that 6 3 2 + 63 = 4032 > 4028 63^2+63=4032 > 4028 , so the correct answer is n = 63 \boxed{n=63} .


Note that we barely made it n = 63 n=63 ; if we, instead, had 2018 2018 vertices, then n = 64 n=64 is necessary.

It was a bit hard for me understand this problem (like what it was asking). :(

Milly Choochoo - 6 years, 12 months ago

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What part was hard to understand? I can fix it.

Daniel Liu - 6 years, 12 months ago

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That illustration of yours made it much better. :)

Milly Choochoo - 6 years, 12 months ago

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.

Jon Haussmann - 6 years, 11 months ago

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Okay, edited.

Daniel Liu - 6 years, 11 months ago

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?

Lim Shian - 6 years, 11 months ago

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Yes, at least change the word "edge" in the question. This is a trap!

Ben Truong - 6 years, 11 months ago

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...

Daniel Liu - 6 years, 11 months ago

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.

Calvin Lin Staff - 6 years, 12 months ago

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I'm pretty sure that it is sufficient. Simply divide the plane using the 63 63 lines into 2017 2017 regions (the maximum possible), and place the 2014 2014 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 2014 K_{2014} need not to be spaced evenly apart; they can be anywhere on the plane.

Daniel Liu - 6 years, 12 months ago

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Ah, I missed that condition. I was thinking that it had to be the graph in the image. Thanks!

Calvin Lin Staff - 6 years, 12 months ago

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