This has to be my new favorite number:
Belphegor’s prime is the palindromic prime number 1000000000000066600000000000001, a number which reads the same both backwards and forwards and is only divisible by itself and 1.
A 666 with 13 zeros and a 1 on either side of it, it was named after Belphegor, one of the seven princes of hell because of its uniquely symbolic sequence.
It was recently featured in an Episode of Elementary, which is where I first learned about this beauty! Could you ask for a cooler prime number then this? Time for the devil to change the combination on his luggage! 
This of course makes me wonder what other amazing numbers are out there and the stories behind them. Anyone know of any good books that cover such interesting numbers and their unique histories and significances?
