220

220 is a natural number that follows 219 and precedes 221. In mathematics, 220 factors as 2^2 × 5 × 11. Its positive divisors are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110, and 220, giving it 12 divisors. The sum of its proper divisors is 284, making 220 an abundant number. It is not expressible as a sum of two squares, because its prime factorization includes 11 with an odd exponent, which violates the condition for sums of two squares.

Representations include the Roman numeral CCXX. In common usage, 220 appears in various contexts such as electrical voltage and musical pitch.

In measurement and signals, 220 volts is a common mains voltage in many countries, typically part of a 230-volt nominal system with 50 Hz frequency. In music, 220 Hz corresponds to the note A3, an octave below the standard concert pitch of A4 at 440 Hz.

Other references include the Simple Mail Transfer Protocol (SMTP) status code 220, which indicates that a server is ready for communication. The international calling code +220 is assigned to the Gambia. The year 220 CE marks the early stage of the Three Kingdoms period in China, following the fall of the Han dynasty.