If there exists a rational integer such that, when
,
, and
are positive integers,
then
is the
-adic
residue of
,
i.e.,
is an
-adic
residue of
iff
is solvable for
. Reciprocity theorems relate statements of
the form "
is an
-adic
residue of
"
with reciprocal statements of the form "
is an
-adic residue of
."
The first case to be considered was (the quadratic
reciprocity theorem), of which Gauss gave the first correct proof. Gauss also
solved the case
(cubic reciprocity theorem) using integers of the form
, where
is a root of
and
,
are rational integers. Gauss stated
the case
(biquadratic reciprocity theorem)
using the Gaussian integers.
Proof of -adic
reciprocity for prime
was given by Eisenstein in 1844-50 and by Kummer in 1850-61.
In the 1920s, Artin formulated Artin's reciprocity
theorem, a general reciprocity law for all orders.