Five is the third smallest prime number, the next is seven. Because it can be written as 2^(2^1)+1, five is classified as a Fermat prime.
Five is a factor of 10, so fractions with 5 in the denominator do not yield infinite decimal expansions, unlike most other primes.
While polynomial equations of degree 4 and below can be solved with radicals, equations of degree 5 and higher cannot generally be so solved. This is the AbelRuffini theorem. This is related to the fact that the symmetric group S_{n} is a solvable group for n ≤ 4 and not solvable for n ≥ 5.
While all graphs with 4 or fewer vertices are planar, there exists a graph with 5 vertices which is not planar: K_{5}, the complete graph with 5 vertices.
The Roman numeral for five is V.
In Cantonese, five sounds like the word "not" (symbol 唔). So when five appears in front of a lucky number, e.g. 58, the result is considered unlucky.
Giving five refers to slapping hands with someone as a sign of approbation.
Five is also:
For the UK TV channel Five, see Five (TV).
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