Belphegor's prime is the name for the number 1000000000000066600000000000001 = \(10^{30} + 666 \cdot 10^{14} + 1\). It is a palindromic prime, notable for its curious decimal expansion containing the beast number. Clifford Pickover named the number after a demon from the Judeo-Christian mythos, playing on 666's association with the devil in numerology.