84 (number)

In mathematics

• the sum of the first seven triangular numbers.
• the sum of a twin prime.
• a semiperfect number, being thrice a perfect number.
• a palindromic number and a repdigit in bases 11, 13, 20, 27, and 41.
• the lim sup of the largest finite subgroup of the mapping class group of a genus g surface divided by g.

In astronomy

• Messier object M84, a magnitude 11.0 lenticular galaxy in the constellation Virgo
• The New General Catalogue NGC 84, a single star in the constellation Andromeda

  In other fields

• 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" starring Anne Bancroft and Anthony Hopkins
• KKNX Radio 84 in Eugene, Oregon
• The B-Side to "Up All Night"
• British Army term for the 84mm Carl Gustav recoilless rifle.
• How many Earth years it takes Uranus to orbit the sun once