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Geometry Difficulty 6.5 National olympiad Find the answer

We know that 2021=43×472021=43 \times 47. Is there a polyhedron whose surface can be formed by gluing together 43 equal non-planar 47-gons? Please justify your answer with a rigorous argument.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

The answer is YES. All we need to do is to construct an example. Let's consider a standard torus T\mathbb{T}, whose points can be represented by two parameters: T={θ,φ:0θ,φ<2π}\mathbb{T}=\{\theta, \varphi: 0 \leq \theta, \varphi<2 \pi\}. One can view the zz-axis as the axis of symmetry of the torus: ((R+rcosφ)cosθ,(R+rcosφ)sinθ,rsinφ)((R+r \cos \varphi) \cos \theta,(R+r \cos \varphi) \sin \theta, r \sin \varphi). For 1k431 \leq k \leq 43, we consider the following region on the torus Dk={θ,φ:2(k1)43π+3φ86θ2k43π+3φ86}D_{k}=\left\{\theta, \varphi: \frac{2(k-1)}{43} \pi+3 \frac{\varphi}{86} \leq \theta \leq \frac{2 k}{43} \pi+3 \frac{\varphi}{86}\right\}. Intuitively, what we do here is to divide the torus into 43 equal parts, then cut every part along the circle {φ=0}\{\varphi=0\}, keep one side of the cut while sliding the other side along the circle for certain angle. Now, we deform the circle {φ=0}\{\varphi=0\} into a regular 43-gon whose vertices correspond to θ=2k43π\theta=\frac{2 k}{43} \pi. Then DkD_{k} has four "sides" of (two of which lie on {φ=0}\{\varphi=0\} ), four "corners" (two of which are adjacent vertices of the 43-gon, while the other two are midpoints of two sides, we need then mark the vertex of the 43-gon between these two midpoints). We denote Ck,0=(2(k1)43π,0),Ck,1=(2k43π,0)C_{k, 0}=\left(\frac{2(k-1)}{43} \pi, 0\right), C_{k, 1}=\left(\frac{2 k}{43} \pi, 0\right), Dk,0=(2k+143π,2π),Dk,1=(2k+343π,2π)D_{k, 0}=\left(\frac{2 k+1}{43} \pi, 2 \pi\right), D_{k, 1}=\left(\frac{2 k+3}{43} \pi, 2 \pi\right), Ek=(2k+243π,2π)E_{k}=\left(\frac{2 k+2}{43} \pi, 2 \pi\right). Take another "side" of Dk\partial D_{k}, mark 21 points, e.g. Ak,i=(2(k1)43π+3φ86π,i11π),i=1,,21A_{k, i}=\left(\frac{2(k-1)}{43} \pi+3 \frac{\varphi}{86} \pi, \frac{i}{11} \pi\right), i=1, \ldots, 21. Then rotate around zz-axis by 243π\frac{2}{43} \pi to get another 21 points, denote them by Bk,i,i=1,,21B_{k, i}, i=1, \ldots, 21. Now we join Ck,0Ck,1,Ck,0Ak,1,Ck,1Bk,1,Ak,iAk,i+1,Bk,iBk,i+1,Ak,iBk,i,Ak,iBk,i+1(i=1,,21)C_{k, 0} C_{k, 1}, C_{k, 0} A_{k, 1}, C_{k, 1} B_{k, 1}, A_{k, i} A_{k, i+1}, B_{k, i} B_{k, i+1}, A_{k, i} B_{k, i}, A_{k, i} B_{k, i+1}(i=1, \ldots, 21) and Ak,21Dk,0,Bk,21Dk,1,Ak,21Ek,Bk,21Ek,Dk,0Ek,EkDk,1A_{k, 21} D_{k, 0}, B_{k, 21} D_{k, 1}, A_{k, 21} E_{k}, B_{k, 21} E_{k}, D_{k, 0} E_{k}, E_{k} D_{k, 1}. We get a non-planar 47-gon. Thus we get 43 congruent (the construction above is independent of kk )non-planar 47gons, they can be glue together to form a polyhedron.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.