Seven (7) is a positive integer one more than 6 and one less than 8.

## PropertiesEdit

7 is the fourth prime number (between 5 and 11). It is a Mersenne prime exponent (producing 127), and is itself a Mersenne prime (between 3 and 31).

7 is the fourth Lucas number, coming between 4 and 11.

7 is in a twin prime pair with 5.

7 is widely considered a lucky number. It is also a lucky number in the numerical sense (between 3 and 9).

## In googology Edit

Some googologisms based on 7 are googolseptiplex, supersept and trisept.

In Greek-based number naming systems, 7 is associated with prefix hepta-, and with prefix sept- in Latin systems.

Donald Knuth calls this number **septiad**, and it's equal to using the -yllion system.

Using the gag prefix, seven can be called gagtwo, since A(2,2) = 7.

Seven is the smallest number of sides of a regular polygon that cannot be constructed with a compass and straightedge alone. 7 is the last digit of Graham's number, and in fact it is the last digit of any power tower of 3's where the height of the tower is at least 2.

## In Culture Edit

The New Jersey metal band Symphony X composed and performed the song * Seven* as part of their album,

*Paradise Lost*.