28

Twenty-eight is a natural number following twenty-seven and preceding twenty-nine. In mathematics, 28 is even and composite, with divisors 1, 2, 4, 7, 14, and 28, and prime factorization 2^2 × 7. It is a perfect number, equal to the sum of its proper divisors (1 + 2 + 4 + 7 + 14). It is the second perfect number after 6, with 496 as the next. It is also a triangular number, equal to T7 = 7 × 8 / 2, the seventh triangular number. It is a hexagonal number, equal to H4 = 4 × 7, and is thus both triangular and hexagonal.

In numeral representations, 28 is XXVIII in Roman numerals, 11100 in binary, and 1C in hexadecimal. The decimal digits add up to 10, giving a digital root of 1. Its prime factorization is 2^2 × 7, and its divisors are 1, 2, 4, 7, 14, and 28.

In calendar and science contexts, February has 28 days in common years. The atomic number of nickel is 28, giving Ni as its symbol. In astronomy, Messier 28 denotes a globular cluster in the constellation Sagittarius. In other bases, 28 can be expressed as 1001 in base 3 and remains 1C in base 16.

Overall, 28 is notable for its dual status as a perfect number and as both a triangular and hexagonal number, along with its various simple representations across numeral systems.