7

Properties

 * 7 is the fourth prime number, second Mersenne prime, and second hexagonal number. The binary and ternary representations of 7 are 111 and 21 respectively.


 * A seven sided polygon is called a heptagon, and a seven sided polyhedron is called a heptahedron.
 * 7 is a supersticious number.
 * 7 is a Woodall prime.
 * 7 is the largest known n such that n! + 1 is square