Octic reciprocity

In number theory, octic reciprocity is a reciprocity law relating the residues of 8th powers modulo primes, analogous to the law of quadratic reciprocity.

There is a rational reciprocity law for 8th powers, due to Williams. Define the symbol (x|p)_k to be +1 if x is a k-th power modulo the prime p and -1 otherwise. Let p and q be distinct primes congruent to 1 modulo 8, such that (p|q) = (q|p) = +1. Let p = a² + b² = c² + 2d² and q = A² + B² = C² + 2D², with aA odd. Then

(p|q)_8 = (q|p)_8 = (aB-bA|q)_4 (cD-dC|q)_2 \ .

References

Lemmermeyer, Franz (2000), Reciprocity laws. From Euler to Eisenstein, Springer Monographs in Mathematics, Springer-Verlag, Berlin, pp. 289–316, ISBN 3-540-66957-4, MR 1761696, Zbl 0949.11002
Williams, Kenneth S. (1976), "A rational octic reciprocity law", Pacific Journal of Mathematics 63 (2): 563–570, doi:10.2140/pjm.1976.63.563, ISSN 0030-8730, MR 0414467, Zbl 0311.10004

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