78 (seventy-eight) is the natural number following 77 and followed by 79.

In mathematics

78 is a triangular number, and its factorization makes it a sphenic number. As a multiple of a perfect number, 78 is itself a semiperfect number.

77 and 78 form a Ruth-Aaron pair under the second definition in which repeated prime factors are counted as often as they occur.

Since it is possible to find sequences of 78 consecutive integers such that each inner member shares a factor with either the first or the last member, 78 is an Erdős–Woods number.
Karl A. Dahlke proved in 1989 that 78 heptominos of this shape ( ), at a minimum, are required to fill a rectangle.

In science

• The atomic number of platinum

• The number of chromosomes in canine DNA

In astronomy

In sports

• Buffalo Bills Bruce Smith wore #78

• Cleveland Browns Ethan Kelley wears #78

• Denver Broncos Matt Lepsis wears #78

• Indianapolis Colts Tarik Glenn wears #78

• Kansas City Chiefs Bobby Bell wore #78

• Minnesota Vikings Matt Birk wears #78

• New England Patriots Bruce Armstrong wore #78

• New York Giants Walt Yowarsky wore #78

• New York Jets Garry Puetz wore #78

• Philadelphia Eagles Antone Davis wore #78

• St. Louis Rams Jackie Slater wore #78

• Toronto Argonauts Adriano Belli wears #78

• Bruntsfield Buffalos Jackson Thirgood wears #78

In other fields

78 is also:

• In reference to gramophone records, 78 refers those meant to be spun at 78 revolutions per minute. Compare: LP, 33 1/3 and 45 rpm.

• A typical tarot deck containing the 21 trump cards, the fool and the 56 suit cards make up 78 cards

• The number of the French department Yvelines

• The total number of gifts in the song The Twelve Days of Christmas (since 78 is the 12th triangular number)

• The year AD 78, 78 BC, or 1978

• The number of lines that make up Metatron's Cube

• Many numbered highways in the United States

• Municipal Okrug 78, a municipal okrug of Tsentralny District of St. Petersburg, Russia

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