78 (number)

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

In mathematics

With 3 distinct prime factors 78 is a sphenic number. It is an abundant number with an aliquot sum of 90. As a multiple of a perfect number, 78 is itself a semiperfect number.

78 is the 12th triangular number.

77 and 78 form a Ruth-Aaron pair.

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 heptominoes of this shape ( ), at a minimum, are required to fill a rectangle.

In science

• The atomic number of platinum.

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. 33+45=78
• A typical tarot deck containing the 21 trump cards, the Fool and the 56 suit cards make up 78 cards
• The total number of gifts in the song The Twelve Days of Christmas (since 78 is the 12th triangular number)
• The number of lines that make up Metatron's Cube
• The Rule of 78s is a method of yearly interest calculation
• Municipal Okrug 78, a municipal okrug of Tsentralny District of St. Petersburg, Russia