Number 5 (Five). It is the cardinal of the set {a, b, c, d, e}. Many towns, including the Aymara of Peru and Bolivia, have used the numbering system called quinario as the basis. Which has a zoological motivation, since the man has five fingers on each hand [1] . It is the third prime integer, after 2 and 3.
- It is the smallest Pythagorean number, since it is the sum of 2 2and 1 2
- They are twin cousins with 3. Along with 2, 3 and 7, they make up the only four cousins out of a dozen.
- The dodecahedron is the only one of the five regular polyhedra whose faces are all regular pentagons.
- According to the Abel-Ruffini Theorem, the fifth degree equation is the least degree equation that has no formula for its solution.
- It is the arithmetic mean of two primes: three and seven.
- is the geometric mean of the sequence: 1, 5, 25. [2]
- It is the third odd natural number, being the previous 1 and 3.
- It is the vulgar logarithm of 100,000 = 10 5.
- It is the sum of two prime numbers; since 5 = 2 + 3.
- The additive group of the remainders of the integers modulo 5 has order 5 [3]
- The convex pentagon is the only polygon whose number of sides matches the number of its diagonals. [4]
- The 5is a Gaussian integer; since 5 = 5 + 0i; but since 5 = (2 + i) (2-i) = (1 + 2i) x (1-2i), it is not a prime integer Gaussian, despite being a prime integer
