78 (number)

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

In mathematics
78 is:


 * the 4th discrete tri-prime; or also termed Sphenic number, and the 4th of the form (2.3.r).
 * an abundant number with an aliquot sum of 33; within an aliquot sequence of nine composite numbers (78, 90,144,259,45,33,15,9,4,3,1,0) to the Prime in the 3-aliquot tree.
 * a semiperfect number, as a multiple of a perfect number.
 * the 12th triangular number.
 * a palindromic number in bases 5 (3035), 7 (1417), 12 (6612), 25 (3325), and 38 (2238).
 * a Harshad number in bases 3, 4, 5, 6, 7, 13 and 14.
 * an Erdős–Woods number, 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.
 * the dimension of the exceptional Lie group E6 and several related objects.
 * the smallest number that can be expressed as the sum of four distinct nonzero squares in more than one way: $$8^2+3^2+2^2+1^2$$, $$7^2+4^2+3^2+2^2$$ or $$6^2+5^2+4^2+1^2$$ (see image).

77 and 78 form a Ruth–Aaron pair.

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 Rule of 78s is a method of yearly interest calculation
 * The number used by Martin Truex Jr. and Furniture Row Racing to win the 2017 Monster Energy NASCAR Cup Series championship and 2016 Coca-Cola 600. The team and driver Regan Smith also won the 2011 Showtime Southern 500 with 78. The number is now used by owner-driver B.J. McLeod for Live Fast Motorsports.