In mathematics, a Schur-convex function, also known as S-convex, isotonic function and order-preserving function is a function that for all such that is majorized by , one has that . Named after Issai Schur, Schur-convex functions are used in the study of majorization.
A function f is 'Schur-concave' if its negative, −f, is Schur-convex.
Every function that is convex and symmetric (under permutations of the arguments) is also Schur-convex.
Every Schur-convex function is symmetric, but not necessarily convex.[1]
If is (strictly) Schur-convex and is (strictly) monotonically increasing, then is (strictly) Schur-convex.
If is a convex function defined on a real interval, then is Schur-convex.
If f is symmetric and all first partial derivatives exist, then f is Schur-convex if and only if
holds for all .[2]