## 26 May 2017

### Seven

$7$ is a prime number.

$7$ is a Mersenne number, since it is prime and $7=2^3-1$. It is also a double Mersenne number, since $3$ is itself a Mersenne number ($3=2^2-1$).

$7$ is the number of pieces of the Tangram.

$7$ is the number of notes in the diatonic scale.

The heptagon is a polygon with $7$ sides.

There are $7$ days a week.