Sard's theorem

In mathematics, Sard's theorem, also known as Sard's lemma or the Morse–Sard theorem, is a result in mathematical analysis that asserts that the set of critical values (that is, the image of the set of critical points) of a smooth function f from one Euclidean space or manifold to another is a null set, i.e., it has Lebesgue measure 0.

This makes the set of critical values "small" in the sense of a generic property.

The theorem is named for Anthony Morse and Arthur Sard.

times continuously differentiable), where

denote the critical set of

which is the set of points

has Lebesgue measure 0 in

Intuitively speaking, this means that although

may be large, its image must be small in the sense of Lebesgue measure: while

may have many critical points in the domain

, it must have few critical values in the image

More generally, the result also holds for mappings between differentiable manifolds

The critical set

function consists of those points at which the differential has rank less than

, then Sard's theorem asserts that the image of

This formulation of the result follows from the version for Euclidean spaces by taking a countable set of coordinate patches.

The conclusion of the theorem is a local statement, since a countable union of sets of measure zero is a set of measure zero, and the property of a subset of a coordinate patch having zero measure is invariant under diffeomorphism.

There are many variants of this lemma, which plays a basic role in singularity theory among other fields.

was proven by Anthony P. Morse in 1939,[2] and the general case by Arthur Sard in 1942.

[1] A version for infinite-dimensional Banach manifolds was proven by Stephen Smale.

[3] The statement is quite powerful, and the proof involves analysis.

In topology it is often quoted — as in the Brouwer fixed-point theorem and some applications in Morse theory — in order to prove the weaker corollary that “a non-constant smooth map has at least one regular value”.

In 1965 Sard further generalized his theorem to state that if

is the set of points

, then the r-dimensional Hausdorff measure of

is at most r. Caveat: The Hausdorff dimension of