A cubic analogue of the jacobsthal identity

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It is well known that if p is a prime such that p = 1 (mod 4), then p can be expressed as a sum of two squares. Several proofs of this fact are known and one of them, due to E. Jacobsthal, involves the identity p = x2 + y2, with x and y expressed explicitly in terms of sums involving the Legendre symbol. These sums are now known as the Jacobsthal sums. In this short note, we prove that if p = 1 (mod 6), then 3p = u2 + uv + v 2 for some integers u and v using an analogue of Jacobsthal 's identity. © THE MATHEMATICAL ASSOCIATION OF AMERICA.

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American Mathematical Monthly

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