400

400 is an integer in the decimal system. It is even and composite, and it is also a perfect square (20 squared). The prime factorization of 400 is 2^4 × 5^2, making it a 5-smooth number. Its divisors, derived from the factors, total 15.

In mathematical properties, 400 is notable for being a square and for its factor structure. It is also a Harshad (or Niven) number in base 10, since it is divisible by the sum of its digits (4 + 0 + 0 = 4; 400 ÷ 4 = 100).

In numeral systems and notation, 400 is represented as CD in Roman numerals, with D representing 500 and C before D indicating 100 less than 500.

In computing, the code 400 Bad Request is an HTTP status indicating that the server could not process the request due to something that is perceived to be a client error, such as malformed syntax or invalid request framing.

In everyday use, 400 is common as a round number, often occurring in contexts like distance (for example, the 400-meter track race) or as a milestone value in statistics and measurements.

As a calendar year, 400 CE falls in late antiquity, a period characterized by various migrations and long-term transformations across Europe and the surrounding regions.

Overall, 400 functions as a versatile benchmark number in mathematics, representation, computing, sports, and historical dating.