Lagrange's four-square theorem states that every natural number can be expressed as the sum of four square numbers. For example, the number 7 can be represented as 4 + 1 + 1 + 1 (where 4 = 2^2 and 1 = 1^2). This theorem was proven by the mathematician Joseph-Louis Lagrange in 1770 and is a significant result in number theory. The theorem implies that no matter how large a number is, you can always find four integers whose squares add up to that number. This result has implications in various areas of mathematics, including algebra and combinatorics.