18

18 = 6 + 6 + 6 DEMONIC (666)

18 = 9 + 9 DIVINE (99)

In speech, the numbers 18 and 80 are sometimes confused. When carefully enunciated, they differ in which syllable is stressed: 18 / vs 80.  However, in dates such as 1864, or when contrasting numbers in the teens, such as 17, 18, 19, the stress shifts to the first syllable: 18. In some dialects, such as General American, there is little confusion because the single t sound in 80 becomes a d-like flap, whereas the double t sound in 18 does not. In a 24-hour clock, the eighteenth hour is in conventional language called six or six o'clock.

 

In mathematics

Eighteen is a composite number, its divisors being 1, 2, 3, 6 and 9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a semiperfect number. Eighteen is the first inverted square-prime of the form p·q2.

It is a heptagonal number and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number.  It is also the sum of the totient function for the first seven integers. It is the sixth lucas number.

Eighteen is the second abundant number with the aliquot sum of 21 (117% in abundance) in the aliquot sequence (18, 21, 11, 1, 0) thus being the first composite number in the 11-aliquot tree. Eighteen is the aliquot sum of only one number the square 289.

In base 10 it is a Harshad number.

18, aside from 0, is the only number that equals twice the sum of its decimal digits.