eightynine

Eightynine refers to the integer 89, commonly written as eighty-nine in standard English. It is a two-digit prime number and is also a Fibonacci number (the 11th term, following 55 and preceding 144). As a decimal number, its digits sum to 17, and its digital root is 8. In Roman numerals, eighty-nine is written as LXXXIX. It is the atomic number of actinium, with the symbol Ac, and in binary it is 1011001; in hexadecimal, it is 0x59.

In mathematics, eightynine is notable as one of the prime Fibonacci numbers and occurs in various identities and sequences. As a prime, it has only two positive divisors: 1 and 89. As a Fibonacci number, it appears in the intersection of two well-known integer sequences.

Eightynine is also used as a stylistic form in branding, media, and popular culture, often appearing as a stylized or condensed name in logos, product lines, or titles. When spelled as eighty-nine, it follows standard English orthography with a hyphen between eighty and nine; the single-word eightynine is less common and typically restricted to branding or digital contexts.

See also 89 (number).