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transversality (Definition)

Transversality is a fundamental concept in differential topology. We say that two smooth submanifolds $ A,B$ of a smooth manifold $ M$ intersect transversely, if at any point $ x\in A\cap B$, we have

$\displaystyle T_x A + T_x B = T_x X, $
where $ T_x$ denotes the tangent space at $ x$, and we naturally identify $ T_x A$ and $ T_x B$ with subspaces of $ T_x X$.

In this case, $ A$ and $ B$ intersect properly in the sense that $ A\cap B$ is a submanifold of $ M$, and

$\displaystyle \mathrm{codim}(A\cap B) = \mathrm{codim}(A) + \mathrm{codim}(B). $

A useful generalization is obtained if we replace the inclusion $ A\hookrightarrow M$ with a smooth map $ f:A\to M$. In this case we say that $ f$ is transverse to $ B\subset M$, if for each point $ a\in f^{-1}(B)$, we have

$\displaystyle df_a(T_a A) + T_{f(a)}B = T_{f(a)}M. $
In this case it turns out, that $ f^{-1}(B)$ is a submanifold of $ A$, and
$\displaystyle \mathrm{codim}(f^{-1}(B)) = \mathrm{codim}(B).$

Note that if $ B$ is a single point $ b$, then the condition of $ f$ being transverse to $ B$ is precisely that $ b$ is a regular value for $ f$. The result is that $ f^{-1}(b)$ is a submanifold of $ A$. A further generalization can be obtained by replacing the inclusion of $ B$ by a smooth function as well. We leave the details to the reader.

The importance of transversality is that it's a stable and generic condition. This means, in broad terms that if $ f:A\to M$ is transverse to $ B\subset M$, then small perturbations of $ f$ are also transverse to $ B$. Also, given any smooth map $ A\to M$, it can be perturbed slightly to obtain a smooth map which is transverse to a given submanifold $ B\subset M$.



"transversality" is owned by mathcam. [ full author list (2) | owner history (1) ]
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Also defines:  transversal, transverse, transversally, transversely
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Cross-references: small perturbations, terms, generic, stable, regular value, smooth map, inclusion, submanifold, subspaces, tangent space, point, intersect, smooth manifold, smooth submanifolds, topology
There are 5 references to this entry.

This is version 3 of transversality, born on 2003-03-02, modified 2003-07-25.
Object id is 4071, canonical name is Transversality.
Accessed 7856 times total.

Classification:
AMS MSC57R99 (Manifolds and cell complexes :: Differential topology :: Miscellaneous)

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An error by dspivak on 2006-04-10 13:59:44
"Also, given any smooth map f:A-->M, it can be perturbed slightly to obtain a smooth map which is transverse to a given submanifold B in M."

This of course is not true if dim A + dim B < dim M.

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