This number is a prime.
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Adams was the first with a prime number of letters in his last name, among the Presidents of the United States of America. Obama is the most recent. [Post]
There are 5 topological symmetry types for oriented knots: fully symmetric, reversible, +amphicheiral, -amphicheiral, and asymmetric. Not to be confused with prime knots. [Post]
The concept of a regular polyhedron of index two was introduced in Wills for orientable polyhedra with planar faces. There are exactly 5 such polyhedra. [Post]