   7 (seven;) is the natural number following  and preceding . ==mathematics== * seven, the fourth prime number, is not only a mersenne prime (since 23&nbsp;â&nbsp;1&nbsp;=&nbsp;7) but also a double mersenne prime since the exponent, 3, is itself a mersenne prime. it is also a newmanâshanksâwilliams prime, a woodall prime, a factorial prime, a lucky prime, a happy number (happy prime), a safe prime (the only mersenne safe prime) and the fourth heegner number. * seven is the lowest natural number that cannot be represented as the sum of the squares of three integers. (see lagrange's four-square theorem#historical development.) * seven is the aliquot sum of one number, the cubic number  and is the base of the 7-aliquot tree. * n = 7 is the first natural number for which the next statement does not hold: "two nilpotent endomorphisms from cn with the same minimal polynomial and the same rank are similar." * 7 is the only number d for which the equation 2n-d=x2 has more than two solutions for n and x natural. in particular, the equation 2n-7=x2 is known as the ramanujanânagell equation. * 7 is the only dimension, besides the familiar 3, in which a vector cross product can be defined. * 7 is the lowest dimension of a known exotic sphere, although there may exist as yet unknown exotic smooth structures on the 4-dimensional sphere. * 999,999 divided by 
Integers Luck Numerology Superstitions_about_numbers