Paper Punches puzzle!

Geometry Level 4

Consider a paper punch that can be centered at any point of the plane and that, when operated, removes from the plane precisely those points whose distance from the center is irrational. How many punches are needed to remove every point?


The answer is 3.

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1 solution

Daniel Liu
Jul 1, 2015

Note that punching the plane at a center will leave only the points that lie on the circles concentric with center the punching point and radii the rational numbers.

So clearly, 2 2 punches is not sufficient since the intersection of the two sets of concentric circles still leaves points that are not removed. But is it possible to remove all points with three punches?

I claim that punching the plane at the origin and ( ± π , 0 ) (\pm \pi,0) will remove all points.

Suppose that it doesn't. Then a point P ( x , y ) P(x,y) satisfies that the distance between P P and the three punch centers are all rational.

So x 2 + y 2 = p 1 q 1 \sqrt{x^2+y^2}=\dfrac{p_1}{q_1} ( x π ) 2 + y 2 = p 2 q 2 \sqrt{(x-\pi)^2+y^2}=\dfrac{p_2}{q_2} ( x + π ) 2 + y 2 = p 3 q 3 \sqrt{(x+\pi)^2+y^2}=\dfrac{p_3}{q_3} for integers p k , q k p_k, q_k .

Squaring and adding the last two equations gives x 2 + y 2 + 2 π 2 = p 2 2 q 2 2 + p 3 2 q 3 2 x^2+y^2 +2\pi^2= \dfrac{p_2^2}{q_2^2}+\dfrac{p_3^2}{q_3^2}

Squaring the first equation gives x 2 + y 2 = p 1 2 p 2 2 x^2+y^2=\dfrac{p_1^2}{p_2^2}

Substituting x 2 + y 2 x^2+y^2 and simplifying gives 2 π 2 = p 2 2 q 2 2 + p 3 2 q 3 2 p 1 2 p 2 2 2\pi^2=\dfrac{p_2^2}{q_2^2}+\dfrac{p_3^2}{q_3^2}-\dfrac{p_1^2}{p_2^2}

But R H S Q RHS\in\mathbb{Q} and L H S ∉ Q LHS\not\in\mathbb{Q} contradiction.

Thus, 3 \boxed{3} punches are necessary and sufficient.

How did you form your claim? And can you generalize it?

vishnu c - 5 years, 11 months ago

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The motivation of my solution was that I wanted to prove that 3 worked. The easiest way is to assume that a certain setup didn't work, then prove that a contradiction happens. The only problem now was to find a setup that worked, and after a bit of experimentation I came up with the above solution.

As for the generalization, I think that in n-space the answer is n+1 punches.

Daniel Liu - 5 years, 11 months ago

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Wait, what? For a 2-D plane, it's 3 punches. So for and N-D plane, shouldn't it be N+1 punches?

No, by generalization, I didn't mean that. I meant, instead of ( 0 , 0 ) ; ( ± π , 0 ) (0,0); (\pm \pi, 0) , what about, ( a , b ) ; ( c , d ) ; ( e , f ) (a,b); (c,d); (e,f) ? Like where a , b Q a,b\in Q and the rest, c , d , e , f c,d,e,f are all transcendental or have irrational squares or something like that.

vishnu c - 5 years, 11 months ago

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@Vishnu C Whoops, I obviously wasn't thinking when I said that.

As for the generalization, the closest I can come up with is that if at least one of the distances between the three points is transcendental, then it is not possible to find a point ( x , y ) (x,y) with rational distance between all three points.

Daniel Liu - 5 years, 11 months ago

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@Daniel Liu Why transcendental? Why not something like 2 4 \sqrt [4] {2} ? Your L.H.S-R.H.S disagreement is a direct consequence of the fact that 2 π 2 2 \pi ^2 is not rational. So if I make a claim similar to yours, except that I choose ( 0 , 0 ) ; ( ± 2 4 , 0 ) (0,0); (\pm \sqrt [4]{2} , 0) and follow your procedure, I'd get to the L.H.S-R.H.S disagreement part all the same, only this time, it'd be: 2 2 = something rational 2 \sqrt 2 = \text{something rational} .

vishnu c - 5 years, 11 months ago

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@Vishnu C Sorry, I meant any number that is non-constructable with a straightedge and compass.

Daniel Liu - 5 years, 11 months ago

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