666 (six hundred [and] sixty-six) is the natural number following 665 and preceding 667.
In Christianity, 666 is referred to in (most manuscripts of) chapter 13 of the Book of Revelation of the New Testament as the "number of the beast."
In mathematics
666 is the sum of the first thirty-six natural numbers, which makes it a triangular number:
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Since 36 is also triangular, 666 is a doubly triangular number. Also, where 15 and 21 are triangular as well, whose squares ( and ) add to 666 and have a difference of .
The number of integers which are relatively prime to 666 is also 216, ; and for an angle measured in degrees, (where here is the golden ratio).
666 is also the sum of the squares of the first seven primes , while the number of twin primes less than is 666.
A prime reciprocal magic square based on in decimal has a magic constant of 666. The twelfth pair of twin primes is (149, 151), with 151 the thirty-sixth prime number.
666 is a Smith number and Harshad number in base ten. The 27th indexed unique prime in decimal features a "666" in the middle of its sequence of digits.
The Roman numeral for 666, DCLXVI, has exactly one occurrence of all symbols whose value is less than 1000 in decreasing order (D = 500, C = 100, L = 50, X = 10, V = 5, I = 1).
In religion
Number of the beast
In the Textus Receptus manuscripts of the New Testament, the Book of Revelation () cryptically asserts 666 to be "the man's number" or "the number of a man" (depending on how the text is translated) associated with the Beast, an antagonistic creature that appears briefly about two-thirds into the apocalyptic vision. Some manuscripts of the original Koine Greek use the symbols χξϛ chi xi stigma (or χξϝ with a digamma), while other manuscripts spell out the number in words.
In modern popular culture, 666 has become one of the most widely recognized symbols for the Antichrist or, alternatively, the devil. Earnest references to the number occur both among apocalypticist Christian groups and in explicitly anti-Christian subcultures. References in contemporary Western art or literature are, more likely than not, intentional references to the Beast symbolism. Such popular references are therefore too numerous to list.
It is common to see the symbolic role of the integer 666 transferred to the numerical digit sequence 6-6-6. Some people take the Satanic associations of 666 so seriously that they actively avoid things related to 666 or the digits 6-6-6. This is known as hexakosioihexekontahexaphobia.
The Number of the Beast is cited as 616 in some early biblical manuscripts, the earliest known instance being in Papyrus 115.
Other occurrences
In the Bible, 666 is the number of talents of gold Solomon collected each year (see and ). In the Bible, 666 is the number of Adonikam's descendants who return to Jerusalem and Judah from the Babylonian exile (see ). Using gematria, Neron Caesar transliterated from Greek into Hebrew short-form spelling, נרון קסר, produces the number 666. The Latin spelling of "Nero Caesar" transliterated into Ktiv haser Hebrew, נרו קסר, produces the number 616. Thus, in the Bible, 666 may have been a coded reference to Nero, who was the Roman emperor from 55 to 68 AD. Though historic protestants such as Andreas Helwig in 1612 proposed the application of the Isopsephy principle to the papal name Vicarius Filii Dei.