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 one. The name Belphegor refers to one of the Seven Princes of Hell, who was charged with helping people make ingenious inventions and discoveries. … The number itself contains superstitious elements that have given it its name: the number 666 at the heart of Belphegor’s Prime is widely associated as being the Number of the Beast, used in symbolism to represent one of the creatures in the Apocalypse or, more commonly, the Devil. This number is surrounded on either side by thirteen zeroes, with thirteen itself long regarded superstitiously as an unlucky number. Also, it in total has 31 digits, which backwards is thirteen.
(There’s far more to 31 that needs discussing at some point, but this might not be the place …)