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  "body": "\n## Statement\n\nEach natural number $n$ is a sum of four squares of integers: $n = a^2 + b^2 + c^2 + d^2$ for some integers $a$, $b$, $c$ and $d$.\n\n## Remarks\n\nFor example, $7 = 4 + 1 + 1 + 1$ and $15 = 9 + 4 + 1 + 1$.\nThree squares are not enough: $7$ is not a sum of three squares, by [[Legendre's three-square theorem|0B64Q7XNE3]].\nBy [[Euler's four-square identity|08WTN6SCQ6]], it is enough to prove the theorem for primes.\nOther proofs use [[Minkowski's theorem|0K387S2PKQ]] or the Hurwitz quaternions.\n\n## History\n\nDiophantus knew examples, and Bachet stated the theorem in his Latin translation of the Arithmetica of 1621.\nLagrange gave the first proof in 1770.\n\n## Explanations\n\n### L1 human\n\nEvery whole number is a sum of at most four perfect squares, if you allow $0$ as a square.\nSome numbers, such as $7$, $15$ and $23$, need all four.\n",
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