Fifty is the smallest number that can be written as the sum of two squares in two distinct ways: 50 = 12 + 72 = 52 + 52. It is also the sum of three squares, 50 = 32 + 42 + 52. It is a Harshad number.
There is no answer to the equation ö(x) = 50, making 50 a nontotient. Nor is there an answer to the equation x - ö(x) = 50, making 50 a noncototient.
Fifty is also:
- The atomic number of tin
- The fifth magic number in physics
- The traditional number of years in a default jubilee period.
- A calibre of ammunition (0.50 inches: see .50 BMG)
- In millimeters, the focal length of the normal lens in 35mm photography.
- The percentage equivalent to one half, so that the phrase "fifty-fifty" commonly expresses something divided equally in half, or an event of probability one half
- In years of marriage, the gold or "golden" wedding anniversary.