Stratification
A decomposition of a (possibly infinite-dimensional) manifold into connected submanifolds of strictly-diminishing dimensions.
Comments
Usually a "stratification" of a space means more than just some decomposition into connected pieces with diminishing dimensions.
Let be a partially ordered set. A
-decomposition of a topological space
is a locally finite collection of subspaces
of
, labelled by the elements of
, such that:
1) if
;
2) is locally closed for all
;
3) ;
4) if , then
(and this is equivalent to
in
).
As an example, consider the subset of given by the inequality
divided into the four pieces
,
,
,
.
Now let be a subset of a smooth manifold
. A stratification of
is a
-decomposition
for some partially ordered set
such that each of the pieces is a smooth submanifold of
.
The stratification is called a Whitney stratification if for every pair of strata
with
the following Whitney's conditions
and
hold. Suppose that a sequence of points
converges to
and a sequence of points
also converges to
. Suppose, moreover, that the tangent planes
converge to some limiting plane
and that the secant lines
converge to some line
(all with respect to some local coordinate system around
in the ambient manifold
). Then
A) ;
B) .
Condition B) implies in fact condition A).
A few facts and theorems concerning Whitney stratifications are as follows. Any closed subanalytic subset of an analytic manifold admits a Whitney stratification, [a5]. In particular, algebraic sets in , i.e. sets given by the vanishing of finitely many polynomials (cf. also Semi-algebraic set), can be Whitney stratified. Whitney stratified spaces can be triangulated, [a4].
References
[a1] | J. Mather, "Notes on topological stability" , Harvard Univ. Press (1970) (Mimeographed notes) |
[a2] | C.G. Gibson, K. Wirthmüller, A.A. du Plessis, E.J.N. Looijenga, "Topological stability of smooth mappings" , Lect. notes in math. , 552 , Springer (1976) |
[a3] | M. Goresky, "Stratified Morse theory" , Springer (1988) |
[a4] | F. Johnson, "On the triangulation of stratified sets and singular varieties" Trans. Amer. Math. Soc. , 275 (1983) pp. 333–343 |
[a5] | H. Hironaka, "Subanalytic sets" , Number theory, algebraic geometry and commutative algebra , Kinokuniya (1973) pp. 453–493 |
[a6] | H. Whitney, "Tangents to an analytic variety" Ann. of Math. , 81 (1965) pp. 496–549 |
[a7] | H. Whitney, "Local properties of analytic varieties" S. Cairns (ed.) , Differentiable and Combinatorial Topology , Princeton Univ. Press (1965) pp. 205–244 |
[a8] | R. Thom, "Propriétés différentielles locales des ensembles analytiques" , Sem. Bourbaki , Exp. 281 (1964/5) |
Stratification. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Stratification&oldid=16887