Sard's theorem, also known as Sard's lemma or the Morse–Sard theorem, is a result in mathematical analysis which asserts that the image of the set of critical points of a smooth function f from one Euclidean space or manifold to another has Lebesgue measure 0 – they form a null set. This makes it "small" in the sense of a generic property.
be , (that is, times continuously differentiable), where . Let denote the critical set of which is the set of points at which the Jacobian matrix of has rank . Then the image 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 .
consists of those points at which the differential
has rank less than as a linear transformation. If , then Sard's theorem asserts that the image of has measure zero as a subset 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. The case was proven by Anthony P. Morse in 1939 (Morse 1939), and the general case by Arthur Sard in 1942 (Sard 1942).
The statement is quite powerful, and the proof is involved analysis. In topology it is often quoted — as in the Brouwer fixed point theorem and some applications in Morse theory — in order to use the weaker corollary that “a non-constant smooth map has a regular value”, and sometimes “...hence also a regular point”.
In 1965 Sard further generalized his theorem to state that if is for and if is the set of points such that has rank less than or equal to , then has Hausdorff dimension at most .
See also 
- Sternberg, Shlomo (1964), Lectures on differential geometry, Englewood Cliffs, NJ: Prentice-Hall, pp. xv+390, MR 0193578, Zbl 0129.13102.
- Morse, Anthony P. (January 1939), "The behaviour of a function on its critical set", Annals of Mathematics 40 (1): 62–70, doi:10.2307/1968544, JSTOR 1968544, MR 1503449.
- Sard, Arthur (1942), "The measure of the critical values of differentiable maps", Bulletin of the American Mathematical Society 48 (12): 883–890, doi:10.1090/S0002-9904-1942-07811-6, MR 0007523, Zbl 0063.06720.
- Sard, Arthur (1965), "Hausdorff Measure of Critical Images on Banach Manifolds", American Journal of Mathematics 87 (1): 158–174, doi:10.2307/2373229, JSTOR 2373229, MR 0173748, Zbl 0137.42501 and also Sard, Arthur (1965), "Errata to Hausdorff measures of critical images on Banach manifolds", American Journal of Mathematics 87 (3): 158–174, doi:10.2307/2373229, JSTOR 2373074, MR 0180649, Zbl 0137.42501.
- Smale, Stephen (1965), "An Infinite Dimensional Version of Sard's Theorem", American Journal of Mathematics 87 (4): 861–866, doi:10.2307/2373250, JSTOR 2373250, MR 0185604, Zbl 0143.35301.