The second Mersenne prime 
, which is itself the exponent of Mersenne
prime 
. It gives rise to the perfect number 
It is a Gaussian
prime, but not an Eisenstein prime, since
it factors as 
, where 
is a primitive cube root of unity. It is the smallest
non-Sophie Germain prime. It is also the smallest
non-Fermat prime, and as such is the smallest number
of faces of a regular polygon (the heptagon)
that is not constructible by straightedge
and compass.
It occurs as a sacred number in the Bible and in various other traditions. In Babylonian numerology it was considered as the perfect number, the only number between 2 and 10 which is not generated (divisible) by any other number, nor does it generate (divide) any other number.
Words referring to number seven may have the prefix hepta-, derived from the Greek 
-) (heptic), or sept-
(septuple), derived from the Latin septem.
