80 (eighty) is the
natural number
In mathematics, the natural numbers are the numbers 0, 1, 2, 3, and so on, possibly excluding 0. Some start counting with 0, defining the natural numbers as the non-negative integers , while others start with 1, defining them as the positive in ...
following
79 and preceding
81.
In mathematics
80 is:
* the sum of
Euler's totient function
In number theory, Euler's totient function counts the positive integers up to a given integer that are relatively prime to . It is written using the Greek letter phi as \varphi(n) or \phi(n), and may also be called Euler's phi function. In ot ...
φ(''x'') over the first sixteen integers.
* a
semiperfect number
In number theory, a semiperfect number or pseudoperfect number is a natural number ''n'' that is equal to the sum of all or some of its proper divisors. A semiperfect number that is equal to the sum of all its proper divisors is a perfect number. ...
, since adding up some subsets of its divisors (e.g.,
1,
4,
5,
10,
20 and
40) gives 80.
* a
ménage number
''Evening Dress'' (, also known as ''Ménage'') is a 1986 French comedy-drama film directed by Bertrand Blier. It was entered into the 1986 Cannes Film Festival where Michel Blanc won the award for Best Actor.
Background
The idea for ''Tenue ...
.
*
palindromic
A palindrome ( /ˈpæl.ɪn.droʊm/) is a word, number, phrase, or other sequence of symbols that reads the same backwards as forwards, such as ''madam'' or '' racecar'', the date " 02/02/2020" and the sentence: "A man, a plan, a canal – Pana ...
in bases 3 (2222
3), 6 (212
6), 9 (88
9), 15 (55
15), 19 (44
19) and 39 (22
39).
* a
repdigit
In recreational mathematics, a repdigit or sometimes monodigit is a natural number composed of repeated instances of the same digit in a positional number system (often implicitly decimal). The word is a portmanteau of "repeated" and "digit".
Ex ...
in bases 3, 9, 15, 19 and 39.
* the sum of the first 4
twin prime
A twin prime is a prime number that is either 2 less or 2 more than another prime number—for example, either member of the twin prime pair or In other words, a twin prime is a prime that has a prime gap of two. Sometimes the term ''twin prime' ...
pairs ((3 + 5) + (5 + 7) + (11 + 13) + (17 + 19)).
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
The 15 puzzle (also called Gem Puzzle, Boss Puzzle, Game of Fifteen, Mystic Square and more) is a sliding puzzle. It has 15 square tiles numbered 1 to 15 in a frame that is 4 tile positions high and 4 tile positions wide, with one unoccupied pos ...
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.
References
External links
*
wiktionary:eighty for ''80'' in other languages.
{{DEFAULTSORT:80 (Number)
Integers