A 7

7 is a natural number following 6 and preceding 8. It is a prime number, and is often considered lucky in various cultures around the world.

Properties

7 is the fourth prime number, and is not only a Mersenne prime (since 2^3 − 1 = 7) but also a double Mersenne prime as it is itself the exponent for another Mersenne Prime (2^7 − 1 = 127). It is also a Newman–Shanks–Williams prime, a Woodall prime, a factorial prime, a lucky prime, a happy number, a safe prime, and the fourth Heegner number.

In Mathematics

Seven is the lowest natural number that cannot be represented as the sum of the squares of three integers. 7 is the only dimension, besides the familiar 3, in which a vector cross product can be defined.

In Culture

In many cultures around the world, 7 is considered a lucky number. This can be seen in instances such as seven days of the week, seven colors of the rainbow, seven notes on a musical scale, seven seas, and seven continents.

In Science

In the periodic table, seven is the atomic number of nitrogen. In astronomy, the Big Dipper constellation has seven stars. The pH value of pure water is approximately 7, making it neither acidic nor basic.

See Also

• Number theory
• Mathematics
• Culture
• Science