The set of "critical values" of a map 
of map class 
has Lebesgue
measure 0 in 
.
Sard's Theorem
See also
Lebesgue Measure, Map Class, Transversal IntersectionExplore with Wolfram|Alpha
References
Iyanaga, S. and Kawada, Y. (Eds.). Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press, p. 682, 1980.Referenced on Wolfram|Alpha
Sard's TheoremCite this as:
Weisstein, Eric W. "Sard's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SardsTheorem.html