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

There is a rational reciprocity law for 8th powers, due to Williams. Define the symbol ${\displaystyle \left({\frac {x}{p}}\right)_{k}}$ 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 ${\displaystyle \left({\frac {p}{q}}\right)_{4}=\left({\frac {q}{p}}\right)_{4}=+1.}$ Let p = a + b = c + 2d and q = A + B = C + 2D, with aA odd. Then

${\displaystyle \left({\frac {p}{q}}\right)_{8}\left({\frac {q}{p}}\right)_{8}=\left({\frac {aB-bA}{q}}\right)_{4}\left({\frac {cD-dC}{q}}\right)_{2}\ .}$

See also

• Artin reciprocity
• Eisenstein reciprocity

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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