80 (number)

80 (eighty) is the natural number following 79 and preceding 81.

In mathematics 80 is:

The sum of Euler's totient function φ(x) over the first sixteen integers: 1 + 1 + 2 + 2 + 4 + 2 + 6 + 4 + 6 + 4 + 10 + 4 + 12 + 6 + 8 + 8 = 80. A semiperfect number, since adding up some subsets of its divisors (e.g., 1, 4, 5, 10, 20, and 40) gives 80. A ménage number. Palindromic in bases 3 (22223), 6 (2126), 9 (889), 15 (5515), 19 (4419), and 39 (2239). A repdigit in bases 3, 9, 15, 19, and 39. The sum of the first four twin prime pairs: (3 + 5) + (5 + 7) + (11 + 13) + (17 + 19) = 80.

The Pareto principle (also known as the 80-20 rule) states that, for many events, roughly 80% of the effects come from 20% of the causes.

Every solvable configuration of the 15 puzzle can be solved in no more than 80 single-tile moves.A. Brüngger, A. Marzetta, K. Fukuda, and J. Nievergelt, The parallel search bench ZRAM and its applications, Annals of Operations Research 90 (1999), pp. 45–63.

In trade In Northern Europe, there have traditionally been words for eighty units - often eggs or herrings: in German, ol in Danish, and øll in Norwegian (all cognates).

References

External links

wiktionary:eighty for 80 in other languages.