84 (number)

84 (eighty-four) is the natural number following 83 and preceding 85.

In mathematics

A hepteract is a seven-dimensional hypercube with 84 penteract 5-faces

84 is:

• the sum of the first seven triangular numbers (making it a tetrahedral number).^[1]
• 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.

A hepteract is a seven-dimensional hypercube with 84 penteract 5-faces.

In astronomy

• 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.^[2] 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.^[3]

In other fields

dial +84 for Vietnam

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,^[4] 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

See also

• List of highways numbered 84

References