U N S O L V E D :
Can every convex polyhedron be cut open and unfolded into a single, flat, nonoverlapping shape?
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Does any number appear exactly 5 times in Pascal's triangle?
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Is the crossing number additive when taking knot sums?
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What is the best way to cut a donut up into tetrahedra?
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Are there are infinitely many primes p such that p + 2 is also prime?
