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Two threes.
Six is the factorial of 3, and the first perfect number apart from 1; it is also the third triangular number.
The first number after 1 which is not a power of a prime number.
It is the only perfect number which is not the sum of consecutive cubes.
It is the second primorial number.
It is the product of 1,2,3 and the sum of 1,2 3 (no other number has such a property).
It is the second automorphic number.
6 is triangular, and its square is also a triangular number.
It is the number of faces or vertices of both the cube and the octahedron, and it is the number of edges of the tetrahedron.
It is the number of different 4-dimensional polytopes.
φ(6)=2,     d(6)=4,     σ(6)=12 .