December 9

The number 47.

47 is a prime number, it is also a safe prime which means that it can expressed as 2p+1, where p is also a prime.

It is also a Thabit number which means it can be expressed by 3 times 2 raised to a power minus 1.

It is also a Carol number, now this one gets a little more complicated. A Carol number is (2 raised to a power then minus 1) squared then minus 2. In this case 2 cubed minus 1, then squared and subtract 2.

47 is also the usual number of strings on a harp.
posted by Peggy or Guy at 9:39 PM