Euler's four-square identity - Wikipedia

PDF) Lagrange's Four-Square Theorem

Table 3 from Extended Lagrange's four-square theorem

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Solved Lagrange's four square theorem says every natural

SOLVED: In the following sequence of problems, we will start the proof of the Four-Square Theorem conjectured in the third century by Diophantus and proven by Lagrange in 1770 (since it took

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Sum of Squares Function -- from Wolfram MathWorld

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Proving the Pythagorean Theorem. Some algebraic and geometric proofs of…, by Michele Diodati, Not Zero

An Independent Quaternion Proof of Jacobi's Four Square Theorem: Zhang, Yuxuan: 9781719171571: : Books