Sard's Theorem

The set of ``critical values'' of a Map $u:\Bbb{R}^n\to\Bbb{R}^n$ of Class $C^1$ has Lebesgue Measure 0 in $\Bbb{R}^n$.

See also Class (Map), Lebesgue Measure

References

Iyanaga, S. and Kawada, Y. (Eds.). Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press, p. 682, 1980.