69 (number)

69 (sixty-nine; ) is the natural number following 68 and preceding 70. An odd number and a composite number, 69 is divisible by 1, 3, 23 and 69. 69 is a semiprime because it is a natural number that is the product of exactly two prime numbers (3 and 23), and an interprime between the numbers of 67 and 71. Because 69 is not divisible by any square number other than 1, it is categorised as a square-free integer. 69 is also a Blum integer since the two factors of 69 are both Gaussian primes. In number theory, 69 is a deficient number, arithmetic number and a congruent number.

The number and its pictograph give its name to the sexual position of the same name. The association of the number with this sex position has resulted in it being associated in meme culture with sex. People knowledgeable of the meme may respond "nice" in response to the appearance of the number, whether intentionally an innuendo or not.

In mathematics
69, spelled sixty-nine and known in Roman numerals as, is the natural number that follows 68 and precedes 70. An odd number, 69 is divisible by 1, 3, 23 and 69. 69 is a composite number, meaning that it is not a prime, and a lucky number because it is a natural number that remains after repeatedly removing every nth number in a sequence of natural numbers, starting from 1. As a natural number that is the product of exactly two prime numbers (a factorisation of 3 x 23), 69 is a semiprime and an interprime between the prime numbers of 67 and 71. The aliquot sum—the total sum of all the divisors of a number, excluding the number itself—of 69 is 27 within the aliquot sequence of numbers where each number is the aliquot sum of the previous number (69, 27, 13, 1, 0). 69 is the third composite number in the 13-aliquot tree, following 27 and 35.

Because 69 is not divisible by any square number other than 1, it is categorised as a square-free integer. 69 is a Blum integer since the two factors of 69 are both Gaussian primes, and an Ulam number—an integer that is the sum of two distinct previously occurring Ulam numbers in a sequence. 69 is a deficient number because the sum of its proper divisors (excluding itself) is less than the number of itself. As an integer for which the arithmetic mean average of its positive divisors is also an integer, 69 is a arithmetic number. 69 is a congruent number—a positive integer that is the area of a right triangle with three rational number sides—and an amenable number. 69 can be expressed as the sum of consecutive positive integers in multiple ways, making it a polite number.

In decimal, 69 is the only natural number whose square ($4,761$) and cube ($328,509$) use every digit from 0–9 exactly once. It is also the largest number whose factorial is less than a googol. On many handheld scientific and graphing calculators, 69! (1.711224524) is the highest factorial that can be calculated due to memory limitations. In its binary expansion of 1000101, 69 is equal to 105 octal, while 105 is equal to 69 hexadecimal (this same property can be applied to all numbers from 64 to 69). In computing, 69 equates to 2120 in ternary (base-3); 153 in senary (base-6); and 59 in duodecimal (base-12).

Visually, 69 is a strobogrammatic number because it looks the same when viewed both right-side and upside down. 69 is a centered tetrahedral number, a figurate number that represents a pyramid with a triangular base and all other points arranged in layers above the base, forming a tetrahedron shape. 69 is also a pernicious number because there is a prime number of 1s when it is written as a binary number, and an odious number as it is a positive integer that has an odd number of 1s in its binary expansion.

In other fields
In chemistry, 69 is the atomic number of thulium, a rare lanthanide (category of metallic elements). In astronomy, the Messier object M69 is a globular cluster in the constellation of Sagittarius; 69 Hesperia is a main-belt asteroid. NGC 69 is the designation given to a barred lenticular galaxy located in the Andromeda constellation. In ASCII, 69 is the decimal for the uppercase E character.

69ing is a sex position wherein each partner aligns themselves to simultaneously achieve oral sex with each other. In reference to this sex act, the number 69 itself has become an Internet meme as an inherently funny number in which users will respond to any occurrence of the number with the word "nice" to draw specific attention to it. This means to humorously imply that the reference to the sex position was intentional. Because of its association with the sex position and resulting meme, 69 has been named "the sex number". In music, the American rapper 6ix9ine (pronounced "six nine") chose the stage name in reference to the sex position as well as the yin-yang symbol.