- Whitney conditions
In
differential topology , a branch ofmathematics , the Whitney conditions are conditions on a pair of submanifolds of a manifold introduced byHassler Whitney in 1965. A finite filtration by closed subsets "F"i of a smooth manifold such that the difference between successive members "F"i" and "F"(i-1)" of the filtration is either empty or a smooth submanifold of dimension "i", is called a stratification. The connected components of the difference "F"i - "F"(i-1) are the strata of dimension "i". A stratification is called a Whitney stratification if all pairs of strata satisfy the Whitney conditions A and B, as defined below.The Whitney conditions in Rn
Let "X" and "Y" be two disjoint locally closed submanifolds of R n , of dimensions "i" and "j".
* "X" and "Y" satisfy Whitney's condition A if whenever a sequence of points "x"1, "x"2, ... in "X" converges to a point "y" in "Y", and the sequence of tangent "i"-planes "T"m, to "X" at the points "xm" converges to an "i"-plane "T" as "m" tends to infinity, then "T" contains the tangent "j"-plane to "Y" at "y".
* "X" and "Y" satisfy Whitney's condition B if for each sequence "x"1, "x"2, ... of points in "X" and each sequence "y"1, "y"2, ... of points in "Y", each converging to the same point "y" in "Y", such that the sequence of secant lines "Lm" between "xm" and "ym" converges to a line "L" as "m" tends to infinity, and the sequence of tangent "i"-planes "T"m, to "X" at the points "xm" converges to an "i"-plane "T" as "m" tends to infinity, then "L" is contained in "T".
John Mather first pointed out that "Whitney's condition B" implies "Whitney's condition A" in the notes of his lectures at Harvard in 1970, which have been widely distributed. He also defined the notion of Thom-Mather stratified space, and proved that every Whitney stratification is a Thom-Mather stratified space and hence is atopologically stratified space . Another approach to this fundamental result was given earlier byRené Thom in 1969.David Trotman showed in his 1978 Warwick thesis that a stratification of a closed subset in a smooth manifold "M" satifies "Whitney's condition A" if and only if the subspace of the space of smooth mappings from a smooth manifold "N" into "M" consisting of all those maps which are transverse to all of the strata of the stratification, is open (using the Whitney, or strong, topology). The subspace of mappings transversal to any countable family of submanifolds of "M" is always dense by Thom's transversality theorem. The density of the set of transversal mappings is interpreted by saying that transversality is a 'generic' property for smooth mappings, while the openness is interpreted by saying that the property is 'stable'.The reason that Whitney conditions have become so widely used is because Whitney himself proved that every algebraic variety, or indeed analytic variety, admits a Whitney stratification ,i.e. a partition into smooth submanifolds satisfying the Whitney conditions. More general singular spaces, like semialgebraic sets (
Rene Thom ) and subanalytic sets (Heisuke Hironaka ) can be stratified as Whitney stratifications. This leads to their use in engineering and robotics.ee also
*
Whitney stratified space
*Thom-Mather stratified space
*Topologically stratified space References
* Mather, John "Notes on topological stability", Harvard, 1970 (available on his webpage at Princeton University).
* Thom, René "Ensembles et morphismes stratifiés", Bulletin of the American Mathematical Society Vol. 75, pp. 240-284), 1969.
* Trotman, David "Stability of transversality to a stratification implies Whitney (a)-regularity," Inventiones Mathematicae 50(3), pp. 273--277, 1979.
* Trotman, David "Comparing regularity conditions on stratifications," Singularities, Part 2 (Arcata, Calif., 1981), volume 40 of Proc. Sympos. Pure Math., pp. 575--586. American Mathematical Society, Providence, R.I., 1983.
* Whitney, Hassler "Local properties of analytic varieties." Differential and Combinatorial Topology (A Symposium in Honor of Marston Morse) pp. 205--244 Princeton Univ. Press, Princeton, N. J., 1965.
* Whitney, Hassler, "Tangents to an analytic variety," Annals of Mathematics 81, no. 3 (1965), pp. 496--549.
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