For 10 points, give the number of sides of a heptagon.
The discovery of exotic spheres of this dimension earned John Milnor the 2011 Abel prize. This number, the smallest double Mersenne prime, is also the fewest number of sides of a regular polygon that cannot be constructed with a compass and straightedge. This is the largest number of dimensions for which a vector-valued cross product can be defined; the cross product in this many dimensions is related to the octonions. The interior angles of a regular polygon with this many sides equals five pi radians over this number, or around 128 degrees. For 10 points, give the number of sides of a heptagon.
The discovery of exotic spheres of this dimension earned John Milnor the 2011 Abel prize.
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