In mathematical analysis, Hölder's inequality, named after Otto Hölder, is a fundamental inequality relating L^p spaces: let S be a measure space, let 1 ≤ p, q ≤ ∞ with 1/p + 1/q = 1, let f be in L^p(S) and g be in L^q(S). Then fg is in L¹(S) and

By choosing S to be the set {1,...,n} with the counting measure, we obtain as a special case the inequality

valid for all real (or complex) numbers x₁,...,x_n, y₁,...,y_n. By choosing S to be the natural numbers with the counting measure, one obtains a similar inequality for infinite series.

For p = q = 2, we get the Cauchy-Schwarz inequality.

Hölder's inequality is used to prove the generalization of the triangle inequality in the space L^p, the Minkowski inequality, and also to establish that L^p is dual to L^q.