Concept
In algebraic number theory the n-th power residue symbol (for an integer n > 2) is a generalization of the (quadratic) Legendre symbol to n-th powers. These symbols are used in the statement and proof of cubic, quartic, Eisenstein, and related higher reciprocity laws. Let k be an algebraic number field with ring of integers that contains a primitive n-th root of unity Let be a prime ideal and assume that n and are coprime (i.e. .) The norm of is defined as the cardinality of the residue class ring (note that since is prime the residue class ring is a finite field): An analogue of Fermat's theorem holds in If then And finally, suppose These facts imply that is well-defined and congruent to a unique -th root of unity This root of unity is called the n-th power residue symbol for and is denoted by The n-th power symbol has properties completely analogous to those of the classical (quadratic) Legendre symbol ( is a fixed primitive -th root of unity): In all cases (zero and nonzero) The n-th power residue symbol is related to the Hilbert symbol for the prime by in the case coprime to n, where is any uniformising element for the local field . The -th power symbol may be extended to take non-prime ideals or non-zero elements as its "denominator", in the same way that the Jacobi symbol extends the Legendre symbol. Any ideal is the product of prime ideals, and in one way only: The -th power symbol is extended multiplicatively: For then we define where is the principal ideal generated by Analogous to the quadratic Jacobi symbol, this symbol is multiplicative in the top and bottom parameters. If then Since the symbol is always an -th root of unity, because of its multiplicativity it is equal to 1 whenever one parameter is an -th power; the converse is not true. If then If then is not an -th power modulo If then may or may not be an -th power modulo The power reciprocity law, the analogue of the law of quadratic reciprocity, may be formulated in terms of the Hilbert symbols as whenever and are coprime.