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, since adding up some subsets of its divisors (e.g.,
1,
4,
5,
10,
20 and
40) gives 80.
* a
ménage number.
*
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 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 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