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|Factorization||22× 3 × 7|
|Divisors||1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84|
- the sum of the first seven triangular numbers (making it a tetrahedral number).
- the sum of a twin prime (41 + 43).
- a semiperfect number, being thrice a perfect number.
- a palindromic number and a repdigit in bases 11 (7711), 13 (6613), 20 (4420), 27 (3327), and 41 (2241).
- the lim sup of the largest finite subgroup of the mapping class group of a genus g surface divided by g.
- Messier object M84, a magnitude 11.0 lenticular galaxy in the constellation Virgo
- The New General Catalogue object NGC 84, a single star in the constellation Andromeda
- The Saros number of the solar eclipse series which began on -181 April 14 and ended on 1099 May 22. The duration of Saros series 84 was 1280.1 years, and it contained 72 solar eclipses. Further, the number of the lunar eclipse series which began on -96 September 13 and ended on 1401 February 28. The duration of Saros series 84 was 1496.5 years, and it contained 84 lunar eclipses.
In other fields
Eighty-four is also:
- The year AD 84, 84 BC, or 1984.
- The number of years in the Insular latercus, a cycle used in the past by Celtic peoples, equal to 3 cycles of the Julian Calendar and to 4 Metonic cycles and 1 octaeteris
- The atomic number of polonium
- The model number of Harpoon missile
- WGS 84 - The latest revision of the World Geodetic System, a fixed global reference frame for the Earth.
- The house number of 84 Avenue Foch
- The number of the French department Vaucluse
- The code for international direct dial phone calls to Vietnam
- The town of Eighty Four, Pennsylvania
- The company 84 Lumber
- The ISBN Group Identifier for books published in Spain
- A variation of the game 42 played with two sets of dominoes.
- The film "84 Charing Cross Road" (1987) starring Anne Bancroft and Anthony Hopkins
- KKNX Radio 84 in Eugene, Oregon
- The B-Side to "Up All Night" (Take That song)
- British Army term for the 84mm Carl Gustav recoilless rifle.
- How many years it takes Uranus to orbit the sun once
- "Sloane's A000292 : Tetrahedral numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29.
- Bede (731). Ecclesiastical History of the English People., 2.2, Latin original. See Easter controversy#Synod of Whitby in 664 and Computus.