twentyeight

Twentyeight is the written name of the integer 28. It is an even, composite number whose prime factorization is 2^2 × 7. The divisors of 28 are 1, 2, 4, 7, 14, and 28; the sum of the proper divisors is 28, making it a perfect number. It is also the fourth triangular number, since 1 + 2 + 3 + 4 + 5 + 6 + 7 equals 28. In number theory, 28 can be expressed as 2^(p−1)·(2^p−1) when 2^p−1 is a Mersenne prime; with p = 3 this yields 28.

In science, 28 is the atomic number of nickel (Ni), a transition metal known for its strength and corrosion resistance. Nickel is widely used in stainless steel, coins, plating, and rechargeable batteries.

In culture and everyday use, February commonly has 28 days in common years, while some months and calendars use the number 28 in various ways. The phrase twenty-eight appears in literature and media, including references in film such as 28 Days Later.

As a numeric unit, 28 has significance in various systems and is notable among small integers for being a perfect number and a triangular number.