In mathematics, two real numbers are called conjugate indices (or Hölder conjugates) if
Formally, we also define as conjugate to and .
Conjugate indices are used in Hölder's inequality, as well as Young's inequality for products; the latter can be used to prove the former. If are conjugate indices, the spaces L<sup>p</sup> and L<sup>q</sup> are dual to each other (see L<sup>p</sup> space).
The following are equivalent characterizations of Hölder conjugates: