28 (number)
28 (
twenty-eight) is the
natural number following
27 and preceding
29.
In mathematics
It is a composite number, its proper divisors being 1, 2, 4, 7 and 14, making it a perfect number and a harmonic divisor number.
Twenty-eight is the second perfect number. As a perfect number, it is related to the Mersenne prime 7, since 22(23 - 1) = 28. The next perfect number is 496.
Twenty-eight is a triangular number and a hexagonal number. Twenty-eight is the sum of the first five prime numbers
It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 2, 8, 10, 18, 28...
Twenty-eight is the ninth and last number in early Indian Magic square of order 3.
In other fields
Twenty-eight is:
- The atomic number of nickel.
- The fourth Magic number (physics).
- The number of days in the shortest month of the Gregorian calendar, February. (Except in leap years, when there are twenty-nine.)
- The common name for the parrot Barnardius zonarius semitorquatus, widely distributed in Western Australia and South Australia. Its call sounds like "wenniate".
- The number of letters in the Danish and Swedish alphabets.
- In neo-Nazi circles, twenty-eight indicates Blood and Honour (28 = BH - B - second letter of the alphabet and H - the eight letter).
- Part of the title of a romantic comedy movie 28 Days, and an unrelated science fiction movie 28 Days Later.
- Part of the name of a well known indie rock band from Melbourne, Australia called 28 Days - the name is unrelated to either of the movies above.
- Historical years: 28 A.D., 28 B.C, or 1928
- The number of Chinese constellations, "Xiu" or "mansions" (a literal translation), equivalent to the 12 western zodiac constellations.
- The number of normal human teeth, not including the third molars (wisdom teeth).
- The number of dominoes in standard domino sets.