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?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
How did you form your claim? And can you generalize it?
Log in to reply
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.
Log in to reply
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 ) , what about, ( a , b ) ; ( c , d ) ; ( e , f ) ? Like where a , b ∈ Q and the rest, c , d , e , f are all transcendental or have irrational squares or something like that.
Log in to reply
@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 ) with rational distance between all three points.
Log in to reply
@Daniel Liu – Why transcendental? Why not something like 4 2 ? Your L.H.S-R.H.S disagreement is a direct consequence of the fact that 2 π 2 is not rational. So if I make a claim similar to yours, except that I choose ( 0 , 0 ) ; ( ± 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 .
Log in to reply
@Vishnu C – Sorry, I meant any number that is non-constructable with a straightedge and compass.
Problem Loading...
Note Loading...
Set Loading...
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 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 ) will remove all points.
Suppose that it doesn't. Then a point P ( x , y ) satisfies that the distance between P and the three punch centers are all rational.
So x 2 + y 2 = q 1 p 1 ( x − π ) 2 + y 2 = q 2 p 2 ( x + π ) 2 + y 2 = q 3 p 3 for integers p k , q k .
Squaring and adding the last two equations gives x 2 + y 2 + 2 π 2 = q 2 2 p 2 2 + q 3 2 p 3 2
Squaring the first equation gives x 2 + y 2 = p 2 2 p 1 2
Substituting x 2 + y 2 and simplifying gives 2 π 2 = q 2 2 p 2 2 + q 3 2 p 3 2 − p 2 2 p 1 2
But R H S ∈ Q and L H S ∈ Q contradiction.
Thus, 3 punches are necessary and sufficient.