Belphegor's prime is the palindromic prime number 1000000000000066600000000000001 (10³⁰ + 666 × 10¹⁴ + 1), a number which reads the same both backwards and forwards and is only divisible by itself and one.

Belphegor's prime was first discovered by Harvey Dubner, a mathematician known for his discoveries of many large prime numbers and prime number forms. For Belphegor's prime in particular, he discovered the prime while determining a sequence of primes it belongs to.

The name Belphegor refers to one of the Seven Princes of Hell, who was charged with helping people make ingenious inventions and discoveries.^[1] "Belphegor's prime" is a name coined by author Clifford A. Pickover.^[2] 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 and is 31 digits in length (thirteen reversed), with thirteen itself long regarded superstitiously as an unlucky number in Western culture.

A Belphegor number is a palindromic number in the form of ${\displaystyle 1(0...)666(0...)1}$. The sequence of the first four Belphegor numbers is:

${\displaystyle 16661,1066601,100666001,10006660001\ldots }$

Dubner noticed that 16661 is a prime number. By adding zeroes directly on both sides of the 666, Dubner found more palindromic prime numbers, including the Belphegor prime, which is second in the sequence. This sequence eventually became the Belphegor primes, named after the number.^[3] The number of zeroes added on the first seven Belphegor primes are as follows:^[4]

${\displaystyle 0,13,42,506,608,2472,2623\ldots }$

As shown, Belphegor's prime appears as the second in this sequence.

In the short scale, this number would be named "one nonillion, sixty-six quadrillion, six hundred trillion one". In the long scale, this number's name would be "one quintillion, sixty-six billiard, six hundred billion one".

• Prime number
• Palindrome

• Belphegor's Prime: 1000000000000066600000000000001 from Clifford Pickover
• Belphegors's Prime and the Belphegor's numbers sequence (A232449) at The On-Line Encyclopedia of Integer Sequences